SOURCE

$('#verification').hide();
$('#login').click(function () {
  if ($(":text").val().length == 0) {
    $('#username').focus();
    $("#errorMessage").html(
      "请您填写手机/邮箱/用户名").css("color","red");
  } else if ($(":password").val().length == 0) {
    $('#password').focus();
    $("#errorMessage").html(
         "请您填写密码").css("color","red");
    $('#verification').show();
  } else if ($('#verification').val().length == 0) {
    $('#verification').focus();
    $('#errorMessage').html("请您填写验证码").css("color","red");
  }
});
$('#verification').blur(function () {
  if ($('#username').val().length != 0 && $('#password').val().length != 0) {

  if ($('#verification').val().length == 0) {
    $('#errorMessage').html("请您填写验证码").css("color","red");
  } else if ($('#verification').val() != 'yzm') {
      $("#errorMessage").html(
      "您输入的验证码有误: " + $('#verification').val()).css("color","red");
  } else {
    $("#errorMessage").html('').css("color","red");
  }
  }
});

<p id="errormessage"></p>
  <input id="username" type="text" name="username"><br>
  <input  id="password" type="password" name="password"><br>
<input type="text" id="verification"/><br>
  <button id="login">登录</button>