html xmlns="http://www.w3.org/1999/xhtml">
head runat="server">
title>/title>
script src="Scripts/jquery-1.4.1.js" type="text/javascript">/script>
script type="text/javascript">
$(function () {
$(".cg2").change(function () {
var id = $(this).attr("id");
var value = $(this).val();
var newid = '#'+id.replace('_1_', '_2_');//把第一列id替换成第二列id
//alert(newid);
var data = "t1*v1|t2*v2|t3*v3";//后台获取的数据格式，这里用固定值了
var datas = data.split('|');//分割成多组
for (var i = 0; i datas.length; i++) {
var d1 = datas[i].split('*');//每组分割成 显示值和真实值
$(newid).append("option value=\""+d1[1]+"\">" + d1[0] + "/option>");
//alert(d1);
}
// alert(id + "|||" + value);
});
})

/script>
/head>
body>
form id="form1" runat="server">
div>为了满足两列，任意多行。后台动态生成下拉框，还要前后列联级的需求。使用js+jquery，用服务器控件+Ajax也不行，老是选择之后就
asp:DropDownList ID="ddl_1_1" CssClass="cg2" runat="server">
asp:ListItem Text="txt1" Value="val1">/asp:ListItem>
asp:ListItem Text="txt1" Value="val1">/asp:ListItem>
asp:ListItem Text="txt1" Value="val1">/asp:ListItem>
/asp:DropDownList>
asp:DropDownList ID="ddl_2_1" runat="server">
/asp:DropDownList>br/>
asp:DropDownList ID="ddl_1_2" CssClass="cg2" runat="server">
asp:ListItem Text="txt2" Value="val2">/asp:ListItem>
asp:ListItem Text="txt2" Value="val2">/asp:ListItem>
asp:ListItem Text="txt2" Value="val2">/asp:ListItem>
/asp:DropDownList>
asp:DropDownList ID="ddl_2_2" runat="server">
/asp:DropDownList>br/>
asp:Button ID="ButtonGet" runat="server" Text="获取" onclick="ButtonGet_Click" />
asp:Label ID="Label1" runat="server" Text="">/asp:Label>
/div>
/form>
/body>
/html>

//后台

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{

}
}

protected void ButtonGet_Click(object sender, EventArgs e)
{
///获取通过js加选择的 ddl_2_1 控件选中的值，显示在Label1上。
Label1.Text = Request["ddl_2_1"].ToString();
}