diff --git a/src/downloader/downloader.rs b/src/downloader/downloader.rs
index a3f3302..31d1734 100644
--- a/src/downloader/downloader.rs
+++ b/src/downloader/downloader.rs
@@ -245,6 +245,17 @@ impl<T: Write + Seek + Send + Sync + ClearFile + GetTargetFileName> DownloaderIn
         self.progress_bar.qload()
     }
 
+    /// Fallback to simple thread mode.
+    pub fn fallback_to_simp(&self) {
+        self.multi.qstore(false);
+        match self.pd.disable_multi() {
+            Ok(_) => {}
+            Err(e) => {
+                println!("{}", e);
+            }
+        };
+    }
+
     #[inline]
     /// Finishes the progress bar and sets a message
     pub fn finish_progress_bar_with_message(&self, msg: impl Into<Cow<'static, str>>) {
diff --git a/src/downloader/pd_file/file.rs b/src/downloader/pd_file/file.rs
index c4e3e95..9c8f53a 100644
--- a/src/downloader/pd_file/file.rs
+++ b/src/downloader/pd_file/file.rs
@@ -1,3 +1,4 @@
+use crate::concat_error;
 use crate::downloader::pd_file::enums::PdFileResult;
 use crate::downloader::pd_file::enums::PdFileStatus;
 use crate::downloader::pd_file::enums::PdFileType;
@@ -137,6 +138,26 @@ impl PdFile {
         Ok(())
     }
 
+    /// Disable multiple thread support
+    pub fn disable_multi(&self) -> Result<(), PdFileError> {
+        if !self.is_downloading() {
+            self.ftype.replace_with2(PdFileType::SingleThread);
+            if !self.is_mem_only() {
+                self.need_saved.qstore(true);
+                let mut f = self.file.get_mut();
+                let f = f.as_mut().unwrap();
+                f.seek(TYPE_OFFSET)?;
+                f.write_le_u8(self.ftype.get_ref().int_value())?;
+                self.need_saved.qstore(false);
+            }
+        } else {
+            let mut err = self.clear();
+            concat_error!(err, self.disable_multi(), PdFileError);
+            return err;
+        }
+        Ok(())
+    }
+
     /// Enable multiple thread support
     pub fn enable_multi(&self) -> Result<(), PdFileError> {
         if !self.is_downloading() {
diff --git a/src/downloader/tasks.rs b/src/downloader/tasks.rs
index 261515e..3298633 100644
--- a/src/downloader/tasks.rs
+++ b/src/downloader/tasks.rs
@@ -157,7 +157,20 @@ pub async fn create_download_tasks_multi_first<
     if status.as_u16() >= 400 {
         return Err(DownloaderError::from(status));
     }
-    // # TODO
+    match result.content_length() {
+        Some(len) => match d.pd.set_file_size(len) {
+            Ok(_) => {}
+            Err(e) => {
+                println!("{}", e)
+            }
+        },
+        None => {
+            d.fallback_to_simp();
+            return Err(DownloaderError::from(gettext(
+                "Warning: no content-length, fallback to single thread download.",
+            )));
+        }
+    }
     Ok(())
 }
 
diff --git a/src/error.rs b/src/error.rs
index 2dcf5f8..7673b3b 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -36,3 +36,22 @@ macro_rules! concat_pixiv_downloader_error {
         }
     };
 }
+
+#[macro_export]
+macro_rules! concat_error {
+    ($exp1:expr, $exp2:expr, $typ:ty) => {
+        $exp1 = match $exp1 {
+            Ok(x) => match $exp2 {
+                Ok(_) => Ok(x),
+                Err(e) => Err(<$typ>::from(e)),
+            },
+            Err(e) => match $exp2 {
+                Ok(_) => Err(e),
+                Err(e2) => {
+                    println!("{}", e);
+                    Err(<$typ>::from(e2))
+                }
+            },
+        }
+    };
+}