<div>
        <select id="sel1" style="width:100px;">
            
        </select>
        <select id="sel2" style="width:100px;">
           
        </select>
        <select id="sel3" style="width:100px;">
            
        </select>
    </div>

 change(1);

    function change(a)
    {
       
        if (a == "1")
        {
            $.ajax({
                url: "select.ashx",
                data: { "code": "0001" },
                type: "post",
                dataType: "json",
                success: function (msg) {
                    $("#sel1").html("");
                    for (i in msg)
                    {
                      
                        var str = "<option value='" + msg[i].code + "'>" + msg[i].name + "</option>";
                        $("#sel1").append(str);
                    }
                    change(2);
                },
                error:function()
                {
                    alert("aaa");
                },
                beforeSend: function () {
                    $("#sel1").html("");
                    var str = "<option value='null'>数据正在加载...</option>";
                    $("#sel1").append(str);
                }

            });
        }
        if (a == "2") {
            $.ajax({
                url: "select.ashx",
                data: { "code": $("#sel1").val() },
                type: "post",
                dataType: "json",
                success: function (msg) {
                    $("#sel2").html("");
                    for (i in msg) {
                        
                        var str = "<option value='" + msg[i].code + "'>" + msg[i].name + "</option>";
                        $("#sel2").append(str);
                    }
                    change(3);
                },
                //如果弹窗跳出，代表服务端路径错误，服务端出错，服务端没有返回指定的json数据格式
                error: function () {
                    alert("aaa");
                },
                //请求服务端的时候执行（不管对错）一开始加载数据就会执行
                beforeSend: function () {
                    $("#sel2").html("");
                    var str = "<option value='null'>数据正在加载...</option>";
                    $("#sel2").append(str);
                },
                //处理完毕之后，不管返回到sussess还是error中   数据加载完毕执行
                complete:function(){
                
                }

            });


        }
        if (a == "3") {
            $.ajax({
                url: "select.ashx",
                data: { "code": $("#sel2").val() },
                type: "post",
                dataType: "json",
                success: function (msg) {
                    $("#sel3").html("");
                    for (i in msg) {
                       
                        var str = "<option value='" + msg[i].code + "'>" + msg[i].name + "</option>";
                        $("#sel3").append(str);
                    }
                },
                error: function () {
                   
                },
                beforeSend: function () {
                    $("#sel3").html("");
                    var str = "<option value='null'>数据正在加载...</option>";
                    $("#sel3").append(str);
                }

            });
        }

    }

    $("#sel1").change(function () {
        change(2);
    });

    $("#sel2").change(function () {
        change(3);
    });

using (cityDataClassesDataContext con = new cityDataClassesDataContext()) 
        {
            StringBuilder str = new StringBuilder();
            str.Append("[");
            string aa = context.Request["code"];
            int count = 0;
            List<ChinaStates> clist = con.ChinaStates.Where(r=>r.ParentAreaCode == aa).ToList();
            foreach(ChinaStates ch in clist )
            {
                if (count > 0) { str.Append(","); }
                str.Append( "{\"name\":\""+ch.AreaName+"\",\"code\":\""+ch.AreaCode+"\"}");
                count++;
            }

            str.Append("]");

            context.Response.Write(str);
            context.Response.End();
        }