jquery中 

$("#<%=StoreName.ClientID %>").focus(function() {

                $("#<%=Label5.ClientID %>").val("请输入真实的店铺名称");

                $("#<%=Label5.ClientID %>").css("color", "#666");

            });

html中

 <table>

                <tr>

                    <td>

                        店铺名称：

                    </td>

                    <td>

                        <asp:TextBox ID="StoreName" runat="server"></asp:TextBox>

                    </td>

                    <td>

                        <asp:Label ID="Label5" runat="server" Text=" 真实的店铺名称，0-10个字。（必填项）"></asp:Label>

                    </td>

                </tr>

            </table>