diff --git a/client/image.go b/client/image.go
index 3806b493..7e1a4f1b 100644
--- a/client/image.go
+++ b/client/image.go
@@ -118,7 +118,7 @@ func (c *QQClient) uploadGroupOrGuildImage(target message.Source, img io.ReadSee
 	if tc > 1 && length > 3*1024*1024 {
 		_, err = c.highwaySession.UploadBDHMultiThread(highway.BdhMultiThreadInput{
 			CommandID: cmd,
-			Body:      utils.ReaderAtFrom2ReadSeeker(img, nil),
+			Body:      img,
 			Size:      length,
 			Sum:       fh,
 			Ticket:    rsp.UploadKey,
diff --git a/client/internal/highway/bdh.go b/client/internal/highway/bdh.go
index 2921d37e..e9605f3c 100644
--- a/client/internal/highway/bdh.go
+++ b/client/internal/highway/bdh.go
@@ -28,7 +28,7 @@ type BdhInput struct {
 
 type BdhMultiThreadInput struct {
 	CommandID int32
-	Body      io.ReaderAt
+	Body      io.ReadSeeker
 	Sum       []byte
 	Size      int64
 	Ticket    []byte
@@ -130,10 +130,11 @@ func (s *Session) UploadBDH(input BdhInput) ([]byte, error) {
 func (s *Session) UploadBDHMultiThread(input BdhMultiThreadInput, threadCount int) ([]byte, error) {
 	// for small file and small thread count,
 	// use UploadBDH instead of UploadBDHMultiThread
-	if input.Size < 1024*1024*3 || threadCount < 2 {
+	// FIXME: enable multi-thread, now receive error code 81
+	if true || input.Size < 1024*1024*3 || threadCount < 2 {
 		return s.UploadBDH(BdhInput{
 			CommandID: input.CommandID,
-			Body:      io.NewSectionReader(input.Body, 0, input.Size),
+			Body:      input.Body,
 			Ticket:    input.Ticket,
 			Ext:       input.Ext,
 			Encrypt:   input.Encrypt,
@@ -207,7 +208,8 @@ func (s *Session) UploadBDHMultiThread(input BdhMultiThreadInput, threadCount in
 			chunk = chunk[:blockSize]
 
 			cond.L.Lock() // lock protect reading
-			n, err := input.Body.ReadAt(chunk, block.Offset)
+			_, _ = input.Body.Seek(block.Offset, io.SeekStart)
+			n, err := input.Body.Read(chunk)
 			cond.L.Unlock()
 
 			if err != nil {
diff --git a/client/ptt.go b/client/ptt.go
index 0d3e7fdf..9ad2caeb 100644
--- a/client/ptt.go
+++ b/client/ptt.go
@@ -150,11 +150,13 @@ func (c *QQClient) UploadShortVideo(target message.Source, video, thumb io.ReadS
 		cmd = 89
 	}
 	ext, _ := proto.Marshal(c.buildPttShortVideoProto(target, videoHash, thumbHash, videoLen, thumbLen).PttShortVideoUploadReq)
+	combined := utils.DoubleReadSeeker(video, thumb)
 	if thread > 1 {
-		sum, _ := utils.ComputeMd5AndLength(utils.MultiReadSeeker(thumb, video))
+		sum, _ := utils.ComputeMd5AndLength(combined)
+		_, _ = combined.Seek(0, io.SeekStart)
 		input := highway.BdhMultiThreadInput{
 			CommandID: cmd,
-			Body:      utils.ReaderAtFrom2ReadSeeker(thumb, video),
+			Body:      combined,
 			Size:      videoLen + thumbLen,
 			Sum:       sum,
 			Ticket:    c.highwaySession.SigSession,
@@ -163,10 +165,9 @@ func (c *QQClient) UploadShortVideo(target message.Source, video, thumb io.ReadS
 		}
 		hwRsp, err = c.highwaySession.UploadBDHMultiThread(input, thread)
 	} else {
-		multi := utils.MultiReadSeeker(thumb, video)
 		input := highway.BdhInput{
 			CommandID: cmd,
-			Body:      multi,
+			Body:      combined,
 			Ticket:    c.highwaySession.SigSession,
 			Ext:       ext,
 			Encrypt:   true,
diff --git a/utils/sys.go b/utils/sys.go
index 7ef22b45..75b54e7d 100644
--- a/utils/sys.go
+++ b/utils/sys.go
@@ -6,9 +6,57 @@ import (
 	"io"
 )
 
-type multiReadSeeker struct {
-	readers     []io.ReadSeeker
-	multiReader io.Reader
+type doubleReadSeeker struct {
+	rs1, rs2       io.ReadSeeker
+	rs1len, rs2len int64
+	pos            int64
+}
+
+func (r *doubleReadSeeker) Seek(offset int64, whence int) (int64, error) {
+	var err error
+	switch whence {
+	case io.SeekStart:
+		if offset < r.rs1len {
+			r.pos, err = r.rs1.Seek(offset, io.SeekStart)
+			return r.pos, err
+		} else {
+			r.pos, err = r.rs2.Seek(offset-r.rs1len, io.SeekStart)
+			r.pos += r.rs1len
+			return r.pos, err
+		}
+	case io.SeekEnd: // negative offset
+		return r.Seek(r.rs1len+r.rs2len+offset-1, io.SeekStart)
+	default: // io.SeekCurrent
+		return r.Seek(r.pos+offset, io.SeekStart)
+	}
+}
+
+func (r *doubleReadSeeker) Read(p []byte) (n int, err error) {
+	switch {
+	case r.pos >= r.rs1len: // read only from the second reader
+		n, err := r.rs2.Read(p)
+		r.pos += int64(n)
+		return n, err
+	case r.pos+int64(len(p)) <= r.rs1len: // read only from the first reader
+		n, err := r.rs1.Read(p)
+		r.pos += int64(n)
+		return n, err
+	default: // read on the border - end of first reader and start of second reader
+		n1, err := r.rs1.Read(p)
+		r.pos += int64(n1)
+		if r.pos != r.rs1len || (err != nil && errors.Is(err, io.EOF)) {
+			// Read() might not read all, return
+			// If error (but not EOF), return
+			return n1, err
+		}
+		_, err = r.rs2.Seek(0, io.SeekStart)
+		if err != nil {
+			return n1, err
+		}
+		n2, err := r.rs2.Read(p[n1:])
+		r.pos += int64(n2)
+		return n1 + n2, err
+	}
 }
 
 func ComputeMd5AndLength(r io.Reader) ([]byte, int64) {
@@ -18,82 +66,19 @@ func ComputeMd5AndLength(r io.Reader) ([]byte, int64) {
 	return fh, length
 }
 
-func (r *multiReadSeeker) Read(p []byte) (int, error) {
-	if r.multiReader == nil {
-		var readers []io.Reader
-		for i := range r.readers {
-			_, _ = r.readers[i].Seek(0, io.SeekStart)
-			readers = append(readers, r.readers[i])
-		}
-		r.multiReader = io.MultiReader(readers...)
-	}
-	return r.multiReader.Read(p)
-}
-
-func (r *multiReadSeeker) Seek(offset int64, whence int) (int64, error) {
-	if whence != 0 || offset != 0 {
-		return -1, errors.New("unsupported offset")
-	}
-	r.multiReader = nil
-	return 0, nil
-}
-
-func MultiReadSeeker(r ...io.ReadSeeker) io.ReadSeeker {
-	return &multiReadSeeker{
-		readers: r,
-	}
-}
-
-type multiReadAt struct {
-	first      io.ReadSeeker
-	second     io.ReadSeeker
-	firstSize  int64
-	secondSize int64
-}
-
-func (m *multiReadAt) ReadAt(p []byte, off int64) (n int, err error) {
-	if m.second == nil { // quick path
-		_, _ = m.first.Seek(off, io.SeekStart)
-		return m.first.Read(p)
-	}
-	if off < m.firstSize && off+int64(len(p)) < m.firstSize {
-		_, err = m.first.Seek(off, io.SeekStart)
-		if err != nil {
-			return
-		}
-		return m.first.Read(p)
-	} else if off < m.firstSize && off+int64(len(p)) >= m.firstSize {
-		_, _ = m.first.Seek(off, io.SeekStart)
-		_, _ = m.second.Seek(0, io.SeekStart)
-		n, err = m.first.Read(p[:m.firstSize-off])
-		if err != nil {
-			return
-		}
-		n2, err := m.second.Read(p[m.firstSize-off:])
-		return n + n2, err
-	}
-	_, err = m.second.Seek(off-m.firstSize, io.SeekStart)
-	if err != nil {
-		return
-	}
-	return m.second.Read(p)
-}
-
-func ReaderAtFrom2ReadSeeker(first, second io.ReadSeeker) io.ReaderAt {
-	firstSize, _ := first.Seek(0, io.SeekEnd)
-	if second == nil {
-		return &multiReadAt{
-			first:      first,
-			firstSize:  firstSize,
-			secondSize: 0,
-		}
-	}
-	secondSize, _ := second.Seek(0, io.SeekEnd)
-	return &multiReadAt{
-		first:      first,
-		second:     second,
-		firstSize:  firstSize,
-		secondSize: secondSize,
+// DoubleReadSeeker combines two io.ReadSeeker into one.
+// input two io.ReadSeeker must be at the start.
+func DoubleReadSeeker(first, second io.ReadSeeker) io.ReadSeeker {
+	rs1Len, _ := first.Seek(0, io.SeekEnd)
+	_, _ = first.Seek(0, io.SeekStart) // reset to start
+	rs2Len, _ := second.Seek(0, io.SeekEnd)
+	_, _ = second.Seek(0, io.SeekStart) // reset to start
+	return &doubleReadSeeker{
+		rs1:    first,
+		rs2:    second,
+		rs1len: rs1Len,
+		rs2len: rs2Len,
+		pos:    0,
 	}
 }