JQ版

var countdown=180; 
function settime(obj) { 
    if (countdown == 0) {
        obj.removeAttribute("disabled");    
        obj.value="获取验证码"; 
        countdown = 180; 
        return;
    } else { 
        obj.setAttribute("disabled", true); 
        obj.value="重新发送(" + countdown + ")"; 
        countdown--; 
    } 
        setTimeout(function() { 
        settime(obj) }
    ,1000) 
}

纯JS版(手机端)

        var countdown=60;
        function settime(id) {
            var obj = document.getElementById(id);
            if (countdown == 0) {
                obj.removeAttribute("disabled");
                obj.innerText="获取验证码";
                countdown = 60;
                return;
            } else {
                obj.setAttribute("disabled", true);
                obj.innerText = "重新发送(" + countdown + ")";
                countdown--;
            }
            setTimeout(function() {
                    settime(id) }
                ,1000)
        }

扩展操作

    var i = s;
    var t = setInterval(function () {
        i--

        if (i == 1) {
            clearInterval(t)
        }
    }, 1000);