window.onload = function() {

var box = document.getElementById('box');

box.onmouseover = function() {

play(100);

}

box.onmouseout = function() {

play(30);

}

var timer = null;//这俩货既不在上面的｛｝里

var alpha = 30;//也不在下面的｛｝里

function play(target) {

clearInterval(timer);

timer = setInterval(function() {

var speed = 0;

if(alpha > target) {

speed = -10;

} else {

speed = 10;

}

if(alpha == target) {

clearInterval(timer);

} else {

alpha += speed;

box.style.opacity = alpha / 100;

}

}, 30);

}

}

window.onload = function(){

var box = document.getElementById('box') ; 

var timer = null ;

box.onmouseover = function(){

play(100);

}

box.onmouseout = function(){

play(30);

}

function play(target){

var alpha = 30 ;

clearInterval(timer);

timer = setInterval(function(){

var speed = 0 ;

if(alpha > target){

speed = -10;

}else{

speed = 10 ;

}

if(alpha == target){

clearInterval(timer);

}else{

alpha += speed;

box.style.opacity = alpha/100;

}

},30);

}

}

第一：

if(alert > target){//这里应该是 alpha

speed = -0.1;

}else{

speed = 0.1 ;

}

第二：

function play(target){

var timer = null ;//这个变量应该声明在方法体外面。

var box = document.getElementById('box') ; 

clearInterval(timer);

第三：

speed的绝对值为0.1 最后鼠标移出div可能会闪屏  建议缩小 然后同时缩小相应倍数计时器的间隔时间 达到效果不变