diff --git a/shadowsocks-csharp/Data/cn.txt b/shadowsocks-csharp/Data/cn.txt
index 22a6478b..80c2bbf7 100644
--- a/shadowsocks-csharp/Data/cn.txt
+++ b/shadowsocks-csharp/Data/cn.txt
@@ -45,7 +45,7 @@ QRCode=二维码
 Local PAC=使用本地 PAC
 Online PAC=使用在线 PAC
 Update Online PAC URL=更新在线 PAC 网址
-Please input PAC Url=请输入 PAC 网址
+Please input PAC Url, e.g. http://autoproxy2pac.appspot.com/pac/proxy/127.0.0.1/1080=请输入 PAC 网址，例如：http://autoproxy2pac.appspot.com/pac/proxy/127.0.0.1/1080
 
 # Messages
 
@@ -70,4 +70,5 @@ No QRCode found. Try to zoom in or move it to the center of the screen.=未发
 Failed to decode QRCode=无法解析二维码
 Failed to update registry=无法修改注册表
 System Proxy On: =系统代理已启用：
-Running: Port {0}=正在运行：端口 {0}
\ No newline at end of file
+Running: Port {0}=正在运行：端口 {0}
+Please input a valid url=请输入一个有效的网址
\ No newline at end of file
diff --git a/shadowsocks-csharp/View/MenuViewController.cs b/shadowsocks-csharp/View/MenuViewController.cs
index 53284975..3a854865 100755
--- a/shadowsocks-csharp/View/MenuViewController.cs
+++ b/shadowsocks-csharp/View/MenuViewController.cs
@@ -515,10 +515,20 @@ namespace Shadowsocks.View
         private void UpdateOnlinePACURLItem_Click(object sender, EventArgs e)
         {
             string origPacUrl = controller.GetConfiguration().pacUrl;
-            string pacUrl = Microsoft.VisualBasic.Interaction.InputBox(
-                I18N.GetString("Please input PAC Url"),
-                I18N.GetString("Update Online PAC URL"),
-                origPacUrl, -1, -1);
+            string pacUrl = null;
+            while (true)
+            {
+                pacUrl = Microsoft.VisualBasic.Interaction.InputBox(
+                    I18N.GetString("Please input PAC Url, e.g. http://autoproxy2pac.appspot.com/pac/proxy/127.0.0.1/1080"),
+                    I18N.GetString("Update Online PAC URL"),
+                    pacUrl == null ? origPacUrl : pacUrl, -1, -1);
+                if (!string.IsNullOrEmpty(pacUrl) && !System.Text.RegularExpressions.Regex.IsMatch(pacUrl,
+                        @"^http(s)?://([a-zA-Z0-9_-]+)(\.[a-zA-Z0-9_-]+)/?", System.Text.RegularExpressions.RegexOptions.IgnoreCase))
+                    MessageBox.Show(I18N.GetString("Please input a valid url"));
+                else
+                    break;
+            }
+
             if (!string.IsNullOrEmpty(pacUrl) && pacUrl != origPacUrl)
             {
                 controller.SavePACUrl(pacUrl);