自己写的三级联动及注意事项，很容易改造的

  

<asp:DropDownList ID="DropDownList1" runat="server">
            </asp:DropDownList>
            <asp:Label ID="Label1" runat="server" style="display:none;" Text=""></asp:Label>
            <asp:DropDownList ID="DropDownList2" runat="server">
                <asp:ListItem Value="0">--请选择--</asp:ListItem>
            </asp:DropDownList>
            <asp:Label ID="Label2" runat="server" style="display:none;" Text=""></asp:Label>
            <asp:DropDownList ID="DropDownList3" runat="server">
                <asp:ListItem Value="0">--请选择--</asp:ListItem>
            </asp:DropDownList>

 注意以下JS的位置，必须放到form的后面，如果在前面，那么document.getElementById("Label1")将得不到值，因为在执行这JS之前根本没有Label1这标签，故浏览器是找不到的

<script type="text/javascript">
    var SmallClassStr = document.getElementById("Label1").innerText;
    SmallClass=SmallClassStr.split(",");
    Small=new Array()
    for (i=0;i<SmallClass.length;i++){
      Small[i]=SmallClass[i].split("-");
    }
    SmallCount=Small.length;


    //此处实现联动
    function changelocation(locationid)
    {
    document.form1.DropDownList2.length = 1; 
    var locationid=locationid;
    var i;
    for (i=0;i < SmallCount; i++)
        {
            if (Small[i][0] == locationid)
            { 
                document.form1.DropDownList2.options[document.form1.DropDownList2.length] = new Option(Small[i][2], Small[i][1]);
            }        
        }
    document.form1.DropDownList3.options.length = 1;
    }
    
    var ThreeStr = document.getElementById("Label2").innerText;
    ThreeClass=ThreeStr.split(",");
    Three=new Array()
    for (i=0;i<ThreeClass.length;i++){
      Three[i]=ThreeClass[i].split("-");
    }
    ThreeCount=Three.length;


    //此处实现联动
    function changelocationa(locationid)
    {
    document.form1.DropDownList3.length = 1; 
    var locationid=locationid;
    var i;
    for (i=0;i < ThreeCount; i++)
        {
            if (Three[i][0] == locationid)
            { 
                document.form1.DropDownList3.options[document.form1.DropDownList3.length] = new Option(Three[i][2], Three[i][1]);
            }        
        }
    }
    </script>

DataTable BigDt = Product.Show_ProductBigClassList();
            DropDownList1.DataSource = BigDt;
            DropDownList1.DataTextField = "BigClassName";
            DropDownList1.DataValueField = "ID";
            DropDownList1.DataBind();
            DropDownList1.Items.Insert(0, new ListItem("--请选择--", "0"));
            DropDownList1.Attributes.Add("onChange", "changelocation(document.form1.DropDownList1.options[document.form1.DropDownList1.selectedIndex].value)");
            DropDownList2.Attributes.Add("onChange", "changelocationa(document.form1.DropDownList2.options[document.form1.DropDownList2.selectedIndex].value)");

            DataTable SmallDt = Product.Show_ProductSmallList();
            foreach (DataRow row in SmallDt.Rows)
            {
                Label1.Text = Label1.Text + row["Pid"] + "-" + row["Id"] + "-" + row["SmallClassName"] + ",";
            }

            DataTable ThreeDt = Product.GetThreeClass();
            foreach (DataRow row in ThreeDt.Rows)
            {
                Label2.Text = Label2.Text + row["ParentId"] + "-" + row["Id"] + "-" + row["ClassName"] + ",";
            }