!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

html xmlns="http://www.w3.org/1999/xhtml">
head>
    title>Ajax/title>
script type="text/javascript">
    function loadXMLDoc(txt) {
        var xmlhttp;
        if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
            xmlhttp = new XMLHttpRequest();
        }
        else {// code for IE6, IE5
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        xmlhttp.onreadystatechange = function () {
            if (xmlhttp.readyState == 4 xmlhttp.status == 200) {
                var citys = xmlhttp.responseText.split(',');
                document.getElementById("citys").length = 1;
                for (i = 0; i citys.length - 1; i++) {

                    document.getElementById("citys").add(new Option(citys[i], citys[i]));
                }
                //                document.getElementById("myDiv").innerHTML = xmlhttp.responseText;
            }
        }
        xmlhttp.open("GET","ajax/getdata.aspx?pro="+txt, true);
        xmlhttp.send();
    }
/script>
/head>
body>

h2>AJAX/h2>
    select id="Select1" onchange="loadXMLDoc(this.value)">
        option>请选择省份/option>
        option value="1">江苏/option>
        option value="2">上海/option>
    /select>
       select id="citys">
        option>请选择城市/option>
    /select>
div id="myDiv">/div>

/body>
/html>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class ajax_getdata : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        string[] js=new string[]{"南京",  "苏州",  "常州" , "无锡" ,  "镇江"};
        string pro=Request.QueryString["pro"];
        if (pro == "1")
        {
            string temp = "";
            for (int i = 0; i js.Length; i++)
            {
                temp = temp + js[i];

                temp = temp + ",";

            }
            Response.Write(temp);
        }
        else
            Response.Write("");
    }
}