按钮发送服务器请求前执行下面js：

<script language="javascript">
                               function validatelead() {
                                   if (document.getElementById("<%=leadTextBox1.ClientID %>").value == "" || document.getElementById("<%=leadTextBox2.ClientID %>").value == "" || document.getElementById("<%=leadTextBox3.ClientID %>").value == "") {
                                       if (document.getElementById("<%=leadTextBox1.ClientID %>").value == "") {
                                           alert("请输入名字");
                                       }
                                       if (document.getElementById("<%=leadTextBox2.ClientID %>").value == "") {
                                           alert("请输入手机");

                                       }
                                       if (document.getElementById("<%=leadTextBox3.ClientID %>").value == "") {
                                           alert("请输入邮箱");

                                       }
                                       return false;
                                   }
                                   return true;
                               }
</script>


<asp:Button ID="Button1" runat="server"
                                    Text="  " class="btn-submit" OnClientClick="return validatelead()" onclick="Button1_Click"/>

注意OnClientClick="return validatelead()"