<asp:Button ID="btnLogin" runat="server" Text="Login" onclick="btnLogin_Click" />

<script type="text/javascript">

        //执行键盘按键命令
        function SubmitKeyClick(e) {
            var keycode = "0";
            //IE浏览器
            if (browser == "Microsoft Internet Explorer") {
                keycode = event.keyCode;
            }
            else {
                //火狐浏览器
                keycode = e.which;
            }
            if (keycode == "13")  //回车键是13
            {
                document.getElementById("ctl00_ContentPlaceHolder1_btnLogin").focus();
                document.getElementById("ctl00_ContentPlaceHolder1_btnLogin").click();
                return;
            }
        }
    </script>