<script language="javascript" type="text/javascript" src="JScripts/jquery-1.8.3.js"></script>
    <script language="javascript" type="text/javascript" src="JScripts/jquery-ui-1.9.2.custom.min.js"></script>
    <link type="text/css" rel="Stylesheet" href="Css/jquery-ui-1.9.2.custom.min.css" />
    <script language="javascript" type="text/javascript">
        var loadingDialog = $('<div id="dialog" title="" width="100%"><p class="center"><img src="Images/loading.gif"/></p><br/><p class="center">Please wait for a little while.</p></div>');
        function bsl_showLoadingDialog(title) {
            loadingDialog.dialog({
                modal: true,title: title,width: 400,height: 300,cloSEOnEscape: false,resizable: false
            });
            loadingDialog.parent().find('.ui-dialog-titlebar-close').hide();
        }

        $(function () {
            $("#btnOK").click(function () {

                $.ajax({
                    type: "POST",async:true,url: "WebForm1.aspx/TestAjax",//必须是后台的静态方法
                    contentType: "application/json; charset=utf-8",dataType: "json",//  data: "name=John&location=Boston",data:"",beforeSend: function () {
                        bsl_showLoadingDialog("请稍等");
                    },success: function (msg) {
                        $(loadingDialog).dialog("close");
                        alert("Data Saved: " + msg.d);
                    },error: function (err) {
                        $(loadingDialog).dialog("close");
                        alert(err);
                    }
                });
                //禁用按钮的提交 
                return false;
            });
        });
    </script>

  <asp:Button ID="btnOK" runat="server" Text="验证用户"  /> 

  [WebMethod]//从前台调用必须是static方法，且必须加WebMethod
        public static string TestAjax()
        {
          //  lblName.Text = "Hello,Async";
            WebForm1 webForm = new WebForm1();
            if (webForm.lblName != null)   //此处的lblName(Label)为null值，这个方法放在WebForm1的类中
            {
                webForm.lblName.Text = "Hello,Async";
            }
           Thread.Sleep(3000);
            return "true";
        }