<form id="form1" runat="server">
    <div>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <div>
                    姓名：<asp:TextBox ID="txtUserName" runat="server" 
                        ontextchanged="txtUserName_TextChanged" AutoPostBack="true"></asp:TextBox>
                    <asp:Label ID="lblErrorMessage" runat="server" Text=""></asp:Label>
                </div>
                <div>
                    密码：<asp:TextBox ID="txtUserPwd" runat="server"></asp:TextBox>
                </div>
                <div>
                    <asp:Button ID="btnOk" runat="server" Text="确定" />
                </div>
            </ContentTemplate>
        </asp:UpdatePanel>
    </div>
    </form>

------解决方案--------------------
写一个JQuery就完啦
$(function(){
  $("#tbox_username").blur(function(){
      $.ajax({
          url:"" //提交到哪个地址去执行
            type：""  //提交方式'post' or 'get'
          data:{"":""} //如果有参数可以用data传递
          success:function(){
             //成功后执行的操作
          }
      })
  })
})

------解决方案--------------------

。。。 <form id="form1" runat="server">
   <div>
       <asp:ScriptManager ID="ScriptManager1" runat="server">
       </asp:ScriptManager>
       <asp:UpdatePanel ID="UpdatePanel1" runat="server">
           <ContentTemplate>
               <div>
                   姓名：<asp:TextBox ID="txtUserName" runat="server"  
                       ontextchanged="txtUserName_TextChanged" AutoPostBack="true"></asp:TextBox>
                   <asp:Label ID="lblErrorMessage" runat="server" Text=""></asp:Label>
               </div>
               <div>
                   密码：<asp:TextBox ID="txtUserPwd" runat="server"></asp:TextBox>
               </div>
               <div>
                   <asp:Button ID="btnOk" runat="server" Text="确定" />
               </div>
           </ContentTemplate>
       </asp:UpdatePanel>
   </div>