<script type="text/javascript" src="img/jquery-1.5.1.min.js"></script>
  <script language="javascript" type="text/javascript">

    var code; //在全局 定义验证码
    var code2; //在全局 定义验证码
    function createCode() {
      code = "";
      var checkCode = document.getElementById("checkCode");
      function RndNum(n) {
        var rnd = "";
        for (var i = 0; i < n; i++)
          rnd += Math.floor(Math.random() * 10);
        return rnd;
      }

      var num = RndNum(2);
      var num2 = RndNum(2);

      code = num + "+" + num2 + "=";
      code2 = parseInt(num) + parseInt(num2)

      if (checkCode) {
        checkCode.className = "code";
        checkCode.value = code;
      }

    }
  </script>
  <script type="text/javascript">
    $(document).ready(function () {

      $("#input1").blur(function () {
        var inputCode = document.getElementById("input1").value;
        if (inputCode.length <= 0) {
          alert("请输入验证码！");
        }
        else if (inputCode != code2) {
          alert("验证码输入错误！");
          createCode(); //刷新验证码
        }
        else {
          alert("^-^ OK");
        }
      });
    })
  </script>

 <form action="#">
   <input type="text" id="input1" />
  <input type="text" onclick="createCode()" readonly="readonly" id="checkCode" class="unchanged" style="width: 80px;background: #660099"/><br />
  </form> 

<style type="text/css">
    .code
    {
      font-family: Arial;
      font-style: italic;
      color: Red;
      border: 0;
      padding: 2px 3px;
      letter-spacing: 3px;
      font-weight: bolder;
    }
    .unchanged
    {
      border: 0;
    }
  </style>