Hey Mengchao,
You can do this with a little piece of Javascript. It will hide the form if today's date is after your designated closed date, and display alternative content:
<script>
MktoForms2.whenReady( function(form) {
var today = new Date(); //set the current date
var registrationClose = new Date("1970/01/01"); //change to your desired date
if (today > registrationClose) {
$(".mktoForm").wrap( "<div class='formHolder'></div>" );
$(".mktoForm").hide();
$(".formHolder").append('<div><p>Sorry! Registration for this event is now closed.</p></div>'); //change to your desired content here, set appropriate classes/IDs for styling
};
});
</script>