diff --git a/okdownload/src/main/java/com/liulishuo/okdownload/DownloadTask.java b/okdownload/src/main/java/com/liulishuo/okdownload/DownloadTask.java
index 5a6eded6..b61defd4 100644
--- a/okdownload/src/main/java/com/liulishuo/okdownload/DownloadTask.java
+++ b/okdownload/src/main/java/com/liulishuo/okdownload/DownloadTask.java
@@ -66,7 +66,8 @@ public class DownloadTask extends IdentifiedTask implements Cloneable, Comparabl
     private final int syncBufferSize;
     private final int syncBufferIntervalMills;
 
-    private final Integer connectionCount;
+    @Nullable private final Integer connectionCount;
+    @Nullable private final Boolean isPreAllocateLength;
 
     /**
      * if this task has already completed with
@@ -96,7 +97,8 @@ public DownloadTask(String url, Uri uri, int priority, int readBufferSize, int f
                         boolean autoCallbackToUIThread, int minIntervalMillisCallbackProcess,
                         Map<String, List<String>> headerMapFields, @Nullable String filename,
                         boolean passIfAlreadyCompleted, boolean isWifiRequired,
-                        Boolean isFilenameFromResponse, Integer connectionCount) {
+                        Boolean isFilenameFromResponse, @Nullable Integer connectionCount,
+                        @Nullable Boolean isPreAllocateLength) {
         this.url = url;
         this.uri = uri;
         this.priority = priority;
@@ -111,6 +113,7 @@ public DownloadTask(String url, Uri uri, int priority, int readBufferSize, int f
         this.passIfAlreadyCompleted = passIfAlreadyCompleted;
         this.isWifiRequired = isWifiRequired;
         this.connectionCount = connectionCount;
+        this.isPreAllocateLength = isPreAllocateLength;
 
         if (Util.isUriFileScheme(uri)) {
             final File file = new File(uri.getPath());
@@ -364,10 +367,20 @@ public int getMinIntervalMillisCallbackProcess() {
      *
      * @return the connection count you set.
      */
-    public Integer getSetConnectionCount() {
+    @Nullable public Integer getSetConnectionCount() {
         return connectionCount;
     }
 
+    /**
+     * Get whether need to pre-allocate length for the file to it's instant-length from trial
+     * connection you set through {@link Builder#setPreAllocateLength(boolean)}.
+     *
+     * @return whether need to pre-allocate length you set.
+     */
+    @Nullable public Boolean getSetPreAllocateLength() {
+        return isPreAllocateLength;
+    }
+
     /**
      * Get the connection count is effect on this task.
      *
@@ -675,6 +688,19 @@ public Builder(@NonNull String url, @NonNull Uri uri) {
 
         private Boolean isFilenameFromResponse;
         private Integer connectionCount;
+        private Boolean isPreAllocateLength;
+
+        /**
+         * Set whether need to pre allocate length for the file after get the resource-length from
+         * trial-connection.
+         *
+         * @param preAllocateLength whether need to pre allocate length for the file before
+         *                          download.
+         */
+        public Builder setPreAllocateLength(boolean preAllocateLength) {
+            isPreAllocateLength = preAllocateLength;
+            return this;
+        }
 
         /**
          * Set the count of connection establish for this task, if this task has already split block
@@ -872,7 +898,7 @@ public DownloadTask build() {
                     syncBufferSize, syncBufferIntervalMillis,
                     autoCallbackToUIThread, minIntervalMillisCallbackProcess,
                     headerMapFields, filename, passIfAlreadyCompleted, isWifiRequired,
-                    isFilenameFromResponse, connectionCount);
+                    isFilenameFromResponse, connectionCount, isPreAllocateLength);
         }
     }
 
diff --git a/okdownload/src/main/java/com/liulishuo/okdownload/core/download/BreakpointLocalCheck.java b/okdownload/src/main/java/com/liulishuo/okdownload/core/download/BreakpointLocalCheck.java
index 30de97d0..32c2e71c 100644
--- a/okdownload/src/main/java/com/liulishuo/okdownload/core/download/BreakpointLocalCheck.java
+++ b/okdownload/src/main/java/com/liulishuo/okdownload/core/download/BreakpointLocalCheck.java
@@ -101,7 +101,7 @@ public boolean isOutputStreamSupportResume() {
         if (supportSeek) return true;
 
         if (info.getBlockCount() != 1) return false;
-        if (OkDownload.with().processFileStrategy().isPreAllocateLength()) return false;
+        if (OkDownload.with().processFileStrategy().isPreAllocateLength(task)) return false;
 
         return true;
     }
diff --git a/okdownload/src/main/java/com/liulishuo/okdownload/core/file/MultiPointOutputStream.java b/okdownload/src/main/java/com/liulishuo/okdownload/core/file/MultiPointOutputStream.java
index 8ced6212..e87fb931 100644
--- a/okdownload/src/main/java/com/liulishuo/okdownload/core/file/MultiPointOutputStream.java
+++ b/okdownload/src/main/java/com/liulishuo/okdownload/core/file/MultiPointOutputStream.java
@@ -89,7 +89,8 @@ public class MultiPointOutputStream {
 
         this.store = store;
         this.supportSeek = OkDownload.with().outputStreamFactory().supportSeek();
-        this.isPreAllocateLength = OkDownload.with().processFileStrategy().isPreAllocateLength();
+        this.isPreAllocateLength = OkDownload.with().processFileStrategy()
+                .isPreAllocateLength(task);
         this.noMoreStreamList = new ArrayList<>();
 
         if (syncRunnable == null) {
diff --git a/okdownload/src/main/java/com/liulishuo/okdownload/core/file/ProcessFileStrategy.java b/okdownload/src/main/java/com/liulishuo/okdownload/core/file/ProcessFileStrategy.java
index 44ff0a3c..9560d989 100644
--- a/okdownload/src/main/java/com/liulishuo/okdownload/core/file/ProcessFileStrategy.java
+++ b/okdownload/src/main/java/com/liulishuo/okdownload/core/file/ProcessFileStrategy.java
@@ -56,8 +56,12 @@ public void discardProcess(@NonNull DownloadTask task) throws IOException {
         return fileLock;
     }
 
-    public boolean isPreAllocateLength() {
+    public boolean isPreAllocateLength(@NonNull DownloadTask task) {
         // if support seek, enable pre-allocate length.
-        return OkDownload.with().outputStreamFactory().supportSeek();
+        boolean supportSeek = OkDownload.with().outputStreamFactory().supportSeek();
+        if (!supportSeek) return false;
+
+        if (task.getSetPreAllocateLength() != null) return task.getSetPreAllocateLength();
+        return true;
     }
 }
diff --git a/okdownload/src/test/java/com/liulishuo/okdownload/DownloadTaskTest.java b/okdownload/src/test/java/com/liulishuo/okdownload/DownloadTaskTest.java
index e77c490a..333e4701 100644
--- a/okdownload/src/test/java/com/liulishuo/okdownload/DownloadTaskTest.java
+++ b/okdownload/src/test/java/com/liulishuo/okdownload/DownloadTaskTest.java
@@ -544,11 +544,20 @@ public void constructor_path() {
                 .build();
 
         // connection count.
+        assertThat(task.getSetConnectionCount()).isNull();
         task = new DownloadTask
                 .Builder("https://jacksgong.com", "not-exist", null)
                 .setConnectionCount(2)
                 .build();
         assertThat(task.getSetConnectionCount()).isEqualTo(2);
+
+        // pre-allocate-length
+        assertThat(task.getSetPreAllocateLength()).isNull();
+        task = new DownloadTask
+                .Builder("https://jacksgong.com", "not-exist", null)
+                .setPreAllocateLength(true)
+                .build();
+        assertThat(task.getSetPreAllocateLength()).isTrue();
     }
 
     @Test
diff --git a/okdownload/src/test/java/com/liulishuo/okdownload/core/download/BreakpointLocalCheckTest.java b/okdownload/src/test/java/com/liulishuo/okdownload/core/download/BreakpointLocalCheckTest.java
index d0c05d66..b08b29ec 100644
--- a/okdownload/src/test/java/com/liulishuo/okdownload/core/download/BreakpointLocalCheckTest.java
+++ b/okdownload/src/test/java/com/liulishuo/okdownload/core/download/BreakpointLocalCheckTest.java
@@ -178,7 +178,7 @@ public void isOutputStreamSupportResume_support() throws IOException {
         when(info.getBlockCount()).thenReturn(1);
         // not pre allocate length
         final ProcessFileStrategy strategy = OkDownload.with().processFileStrategy();
-        when(strategy.isPreAllocateLength()).thenReturn(false);
+        when(strategy.isPreAllocateLength(task)).thenReturn(false);
         assertThat(check.isOutputStreamSupportResume()).isTrue();
     }
 
@@ -195,7 +195,7 @@ public void isOutputStreamSupportResume_notSupport() throws IOException {
         when(info.getBlockCount()).thenReturn(1);
         // pre allocate length but not support seek
         final ProcessFileStrategy strategy = OkDownload.with().processFileStrategy();
-        doReturn(true).when(strategy).isPreAllocateLength();
+        doReturn(true).when(strategy).isPreAllocateLength(task);
         assertThat(check.isOutputStreamSupportResume()).isFalse();
     }
 
diff --git a/okdownload/src/test/java/com/liulishuo/okdownload/core/file/MultiPointOutputStreamTest.java b/okdownload/src/test/java/com/liulishuo/okdownload/core/file/MultiPointOutputStreamTest.java
index 2f7ac4b1..32a59d6a 100644
--- a/okdownload/src/test/java/com/liulishuo/okdownload/core/file/MultiPointOutputStreamTest.java
+++ b/okdownload/src/test/java/com/liulishuo/okdownload/core/file/MultiPointOutputStreamTest.java
@@ -487,7 +487,7 @@ public void outputStream_nonFileScheme() throws IOException {
 
     private void prepareOutputStreamEnv() throws FileNotFoundException, PreAllocateException {
         when(OkDownload.with().outputStreamFactory().supportSeek()).thenReturn(true);
-        when(OkDownload.with().processFileStrategy().isPreAllocateLength()).thenReturn(true);
+        when(OkDownload.with().processFileStrategy().isPreAllocateLength(task)).thenReturn(true);
         when(OkDownload.with().outputStreamFactory().create(any(Context.class), any(Uri.class),
                 anyInt())).thenReturn(mock(DownloadOutputStream.class));
         // recreate for new values of support-seek and pre-allocate-length.
diff --git a/okdownload/src/test/java/com/liulishuo/okdownload/core/file/ProcessFileStrategyTest.java b/okdownload/src/test/java/com/liulishuo/okdownload/core/file/ProcessFileStrategyTest.java
index 51869c2c..f56f9e61 100644
--- a/okdownload/src/test/java/com/liulishuo/okdownload/core/file/ProcessFileStrategyTest.java
+++ b/okdownload/src/test/java/com/liulishuo/okdownload/core/file/ProcessFileStrategyTest.java
@@ -17,8 +17,8 @@
 package com.liulishuo.okdownload.core.file;
 
 import com.liulishuo.okdownload.DownloadTask;
+import com.liulishuo.okdownload.OkDownload;
 
-import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 import org.mockito.Mock;
@@ -26,6 +26,8 @@
 import java.io.File;
 import java.io.IOException;
 
+import static com.liulishuo.okdownload.TestUtils.mockOkDownload;
+import static org.assertj.core.api.Java6Assertions.assertThat;
 import static org.mockito.Mockito.when;
 import static org.mockito.MockitoAnnotations.initMocks;
 
@@ -33,26 +35,49 @@ public class ProcessFileStrategyTest {
     private ProcessFileStrategy strategy;
 
     @Mock private DownloadTask task;
-    private final File existFile = new File("./exist-path");
 
     @Before
     public void setup() throws IOException {
         initMocks(this);
         strategy = new ProcessFileStrategy();
+    }
 
+    @Test
+    public void discardProcess() throws IOException {
+        final File existFile = new File("./exist-path");
         existFile.createNewFile();
-    }
 
-    @After
-    public void tearDown() {
-        existFile.delete();
+        when(task.getFile()).thenReturn(existFile);
+
+        strategy.discardProcess(task);
+
+        assertThat(existFile.exists()).isFalse();
     }
 
     @Test
-    public void discardProcess() throws IOException {
-        when(task.getFile()).thenReturn(new File("mock path"));
+    public void isPreAllocateLength() throws IOException {
+        mockOkDownload();
 
-        strategy.discardProcess(task);
-        // nothing need to test.
+        // no pre-allocate set on task.
+        when(task.getSetPreAllocateLength()).thenReturn(null);
+
+        final DownloadOutputStream.Factory factory = OkDownload.with().outputStreamFactory();
+        when(factory.supportSeek()).thenReturn(false);
+
+        assertThat(strategy.isPreAllocateLength(task)).isFalse();
+        when(factory.supportSeek()).thenReturn(true);
+
+        assertThat(strategy.isPreAllocateLength(task)).isTrue();
+
+        // pre-allocate set on task.
+        when(task.getSetPreAllocateLength()).thenReturn(false);
+        assertThat(strategy.isPreAllocateLength(task)).isFalse();
+
+        when(task.getSetPreAllocateLength()).thenReturn(true);
+        assertThat(strategy.isPreAllocateLength(task)).isTrue();
+
+        // pre-allocate set on task is true but can't support seek.
+        when(factory.supportSeek()).thenReturn(false);
+        assertThat(strategy.isPreAllocateLength(task)).isFalse();
     }
 }
\ No newline at end of file