diff --git a/shadowsocks-csharp/View/QRCodeForm.cs b/shadowsocks-csharp/View/QRCodeForm.cs
index eb009cbf..4537e0f1 100755
--- a/shadowsocks-csharp/View/QRCodeForm.cs
+++ b/shadowsocks-csharp/View/QRCodeForm.cs
@@ -33,6 +33,14 @@ namespace Shadowsocks.View
             QRCode code = ZXing.QrCode.Internal.Encoder.encode(qrText, ErrorCorrectionLevel.M);
             ByteMatrix m = code.Matrix;
             int blockSize = Math.Max(pictureBox1.Height/m.Height, 1);
+
+            var qrWidth = m.Width*blockSize;
+            var qrHeight = m.Height*blockSize;
+            var dWidth = pictureBox1.Width - qrWidth;
+            var dHeight = pictureBox1.Height - qrHeight;
+            var maxD = Math.Max(dWidth, dHeight);
+            pictureBox1.SizeMode = maxD >= 7*blockSize ? PictureBoxSizeMode.Zoom : PictureBoxSizeMode.CenterImage;
+
             Bitmap drawArea = new Bitmap((m.Width*blockSize), (m.Height*blockSize));
             using (Graphics g = Graphics.FromImage(drawArea))
             {