这个是一个webservice方法，返回一个DATASET

[WebMethod(Description="得到市")]
        public DataSet GetChengShi(string  strType)
        {
            string strcon="server=(local);database=lsa_anqiu;uid=sa;pwd=sundun";
            SqlConnection conn=new SqlConnection(strcon);

            string strSelect=string.Format("select  chengshi from chengshi where shengfen='{0}' ",strType);
    
            SqlDataAdapter da=new SqlDataAdapter(strSelect,conn);
            DataSet ds=new DataSet();
            da.Fill(ds);

            return ds;

        }

//得到城市,webservice
    function GetChengShiJ(drp)
    {
    
            //选中的值(省)
            var strS=window.Form1.drpS.value;
            //如果市有值则清空
            var objList = document.getElementById("drpC");
            if (objList.options.length !=0) 
            { 
                for (var iDel=0;iDel<objList.options.length;iDel++)
                {
                    objList.options[iDel]=null ;
                    objList.options.remove(iDel);
                    iDel--;
                }
            }
            
            //增加一空列
            objList.add(document.createElement("OPTION"));
            objList.options[0].text = "";
            objList.options[0].value = "";
            
            
            var xmlData = new ActiveXObject("Msxml.DOMDocument");
            xmlData.async = false
            //调用webservice,返回一个dataset
            var vstr="Service1.asmx/GetChengShi?strType="+strS
            xmlData.load( vstr);
            
            xmlValue = xmlData.getElementsByTagName("chengshi");
            xmlText = xmlData.getElementsByTagName("chengshi");

            
            for (var i=0; i<xmlValue.length; i++)
            {
                objList.add(document.createElement("OPTION"));
                objList.options[i+1].text = xmlText.item(i).text;
                objList.options[i+1].value = xmlValue.item(i).text;
            }
                        
    }

<SELECT id="drpS" runat="server" onchange="GetChengShiJ(this);">
                            <OPTION selected value="广东">广东</OPTION>
                            <OPTION value="湖北">湖北</OPTION>
                            <OPTION value="湖南">湖南</OPTION>
                            <OPTION value=""></OPTION>
                        </SELECT><SELECT id="drpC" runat="server">
                            <OPTION selected></OPTION>
                        </SELECT>

注意，JAVASCRIPT调用WEBSERVICE需要在WEB。CONFIG里设置

<webServices>
      <protocols>
        <add name="HttpSoap" />
        <add name="HttpPost" />
        <add name="HttpGet" />
        <add name="Documentation" />
      </protocols>
    </webServices>