尝试将其添加到表单onsubmit ="return Submitform();"

try this add this to form onsubmit = "return submitform();"

 function submitform(){
    var comment = $("#comment").val();
    var alertid = $("#alertid").val();
    $.ajax({
        type: "POST",
        //url: "ana.php",
        data:{cmt:comment,alert_id:alertid}
    }).done(function( result ) {
        $("#msg").html( result );
    });
    return false;
  }