diff --git a/transport/internet/kcp/kcp.go b/transport/internet/kcp/kcp.go
index 58d4cbe08..9368bb349 100644
--- a/transport/internet/kcp/kcp.go
+++ b/transport/internet/kcp/kcp.go
@@ -279,6 +279,7 @@ func (kcp *KCP) Input(data []byte) int {
 					kcp.update_ack(_itimediff(kcp.current, ts))
 				}
 				kcp.parse_ack(sn)
+				kcp.shrink_buf()
 				if flag == 0 {
 					flag = 1
 					maxack = sn
@@ -300,9 +301,9 @@ func (kcp *KCP) Input(data []byte) int {
 			}
 			kcp.HandleReceivingNext(seg.ReceivinNext)
 			kcp.HandleSendingNext(seg.SendingNext)
+			kcp.shrink_buf()
 		default:
 		}
-		kcp.shrink_buf()
 	}
 
 	if flag != 0 {
diff --git a/transport/internet/kcp/sending.go b/transport/internet/kcp/sending.go
index d2c891377..6a8ca78b4 100644
--- a/transport/internet/kcp/sending.go
+++ b/transport/internet/kcp/sending.go
@@ -53,6 +53,10 @@ func (this *SendingWindow) Clear(una uint32) {
 }
 
 func (this *SendingWindow) Remove(idx uint32) {
+	if this.len == 0 {
+		return
+	}
+
 	pos := (this.start + idx) % this.cap
 	seg := this.data[pos]
 	if seg == nil {
@@ -80,6 +84,9 @@ func (this *SendingWindow) Remove(idx uint32) {
 }
 
 func (this *SendingWindow) HandleFastAck(number uint32) {
+	if this.len == 0 {
+		return
+	}
 
 	for i := this.start; ; i = this.next[i] {
 		seg := this.data[i]