Edit in JSFiddle

$('form').on('submit', function(e) {
    e.preventDefault();
    alert($('#agree').prop('checked') ? "Agree" : "Disagree");
});

<form action="#" method="post">
    <input id="agree" type="checkbox" /><label for="agree">同意並接受註冊協議</label>
    <input id="submit" type="submit" />
</form>