<asp:TextBox ID="username" runat="server"></asp:TextBox>
           <input type="button" name="check" value="唯一性检查" onClick="userCheck()">

<script language="javascript">
                var http_request = false;
                function send_request(url) 
                {//初始化、指定处理函数、发送请求的函数
                http_request = false;
                /**////alert("1");
                //开始初始化XMLHttpRequest对象
                if(window.XMLHttpRequest)
                 { //Mozilla 浏览器
                    http_request = new XMLHttpRequest();
                    if (http_request.overrideMimeType)
                     {//设置MiME类别
                        http_request.overrideMimeType('text/xml');
                       }
                }
                else if (window.ActiveXObject)
                 { // IE浏览器
                        try 
                        {
                            http_request = new ActiveXObject("Msxml2.XMLHTTP");
                        } 
                        catch (e) 
                            {
                                try {
                                        http_request = new ActiveXObject("Microsoft.XMLHTTP");
                                    } catch (e)
                                 {}
                            }
                }
                if (!http_request) { // 异常，创建对象实例失败
                    window.alert("不能创建XMLHttpRequest对象实例.");
                    return false;
                }
                http_request.onreadystatechange = processRequest;
                // 确定发送请求的方式和URL以及是否同步执行下段代码
                http_request.open("GET", url, true);
                http_request.send(null);
            }
            // 处理返回信息的函数
            function processRequest() {
                if (http_request.readyState == 4) { // 判断对象状态
                    if (http_request.status == 200) { // 信息已经成功返回，开始处理信息
                        alert(http_request.responseText);
                    } else { //页面不正常
                        alert("您所请求的页面有异常。");
                    }
                }
            }
    
    function userCheck() 
            {
                
                var username =  document.getElementById("username").value;
                if(username=="") 
                {
                    window.alert("用户名不能为空。");
                    f.username.focus();
                    return false;
                }
               else {
         
                send_request('Back.aspx?username='+username);
            }
        }
</script>

if (Request.QueryString["username"].ToString().Trim() != "")
        {

            Response.Write(Request.QueryString["username"].ToString().Trim());
        }