if(oUl.offsetLeft>0){
oUl.style.left = -oUl.offsetWidth/2+'px';
} 

if (oUl.offsetLeft<-oUl.offsetWidth/2) {
oUl.style.left = 0; 
} 

<div id="box">
<ul>
<li><a href="#"><img src="1.jpg" /></a></li>
<li><a href="#"><img src="2.jpg" /></a></li>
<li><a href="#"><img src="3.jpg" /></a></li>
<li><a href="#"><img src="4.jpg" /></a></li>
</ul>
</div> 

* {margin: 0;padding: 0;}
#box{ width: 480px; height: 110px; border: 1px red solid; margin: 100px auto;overflow: hidden; position: relative; }
#box ul{ position: absolute; left: 0; top: 5px;}
#box ul li{list-style: none; float: left; width: 100px; height: 100px; padding-left: 16px; }
#box ul li img{width: 100px; height: 100px;} 

<script>
window.onload = function(){
var oDiv = document.getElementById('div1');
var oUl = oDiv.getElementsByTagName('ul')[0];
var oLi = oUl.getElementsByTagName('li');
var aA = document.getElementsByTagName('a');
var iSpeed = 10; //让ul开始就具有一个速度走动 
oUl.innerHTML +=oUl.innerHTML; 
oUl.style.width = oLi.length*oLi[0].offsetWidth+'px'; 
aA[0].onclick = function(){
iSpeed = -10;
};
aA[1].onclick = function(){
iSpeed = 10;
};
function fnMove(){
if (oUl.offsetLeft<-oUl.offsetWidth/2) { //负数是因为ul的left是负数 　　　　　　oUl.style.left = 0;
}
else if(oUl.offsetLeft>0){ oUl.style.left = -oUl.offsetWidth/2+'px'; 
} 
oUl.style.left = oUl.offsetLeft+iSpeed+'px'; //整个ul向右移动 
};
var timer = null; 
clearInterval(timer);
timer = setInterval(fnMove,30);
oUl.onmouseover = function(){
clearInterval(timer);
};
oUl.onmouseout = function(){
timer = setInterval(fnMove,30); //当鼠标移开的时候执行这个定时器
};
};
</script>