<script language="javascript">
var code; //在全局 定义验证码
function createCode()
{ //创建验证码函数
 code = "";
 var codeLength =4;//验证码的长度
 var selectChar = new Array(0,1,2,3,4,5,6,7,8,9,'a','b','c','d','e','f','g','h','i','j','k',
  'l','m','n','o','p','q','r','s','t','u','v','w','x','y','z');//所有候选组成验证码的字符，当然也可以用中文的
 for(var i=0;i<codeLength;i++)
 { 
 var charIndex =Math.floor(Math.random()*36);
 code +=selectChar[charIndex]; 
 }
// 设置验证码的显示样式，并显示
 document.getElementById("discode").style.fontFamily="Fixedsys"; //设置字体
 document.getElementById("discode").style.letterSpacing="5px"; //字体间距
 document.getElementById("discode").style.color="#0ab000"; //字体颜色
 document.getElementById("discode").innerHTML=code; // 显示
}
function but()
{//验证验证码输入是否正确
 var val1=document.getElementById("t1").value;
 var val2=code;
 if(val1!=val2){
  alert("验证码错误!");
 document.getElementById('t1').value="";
  }
 }
</script>

 <div class="input">
    <input id="t1" type="text" name="u" placeholder="验证码" onblur="but()" />
    <span id="discode"></span>
    <input type="button" value="换一换" class="c" style="height:20px;"onClick="createCode()">
    </div>