<asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
    
    </div>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" Width="154px" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
                </asp:DropDownList>
这个是前台的 !后台的你自己解决 !!
绝对不会刷新
                <asp:DropDownList ID="DropDownList2" runat="server" Width="152px">
                </asp:DropDownList>
            </ContentTemplate>
        </asp:UpdatePanel>