function getStyle(obj,attr) {
  if(obj.currentStyle){
    return obj.currentStyle[attr];//为了获取IE下的属性值
  }else{
    return window.getComputedStyle(obj,null)[attr];//为了获取W3C浏览器下的属性值
  }
}

function animate(obj,json){
  clearInterval(obj.timer);
  obj.timer = setInterval(function () {
    var flag = true;
    var current = 0;
    for(var attr in json){
      if(attr == 'opacity'){
        current = parseInt(getStyle(obj,attr)*100);
      }else{
        current = parseInt(getStyle(obj,attr));
      };

      var step = (json[attr] - current) / 10;
      step = step > 0 ? Math.ceil(step) : Math.floor(step);
      //先判断属性是否为透明度
      if(attr == 'opacity'){
        //首先判断浏览器是否支持opacity
        if('opacity' in obj.style){
          obj.style.opacity = (current + step) / 100;
        }else{
          obj.style.filter = 'alpha(opacity = ' + (current + step) + ')';
        }
      //再判断属性是否为z-index
      }else if(attr == 'zIndex'){
        obj.style.zIndex = json[attr];
      //最后再判断
      }else{
        obj.style[attr] = current + step + 'px';
      }

      if(current != json[attr]){
        flag = false;
      }
    }

    if(flag){
      clearInterval(obj.timer);
    }
  },5);
}

<div id="box">
  <ul>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
  </ul>
</div>

*{
    margin:0px;
    padding:0px;
  }
  #box{
    width:1300px;
    height:400px;
    margin:100px auto;
    overflow: hidden;
  }
  #box ul{
    height:400px;
    width:1300px;
  }
  #box ul li{
    width:240px;
    height:400px;
    float:left;
    overflow: hidden;
  }

window.onload = function () {
    var box = document.getElementById('box');
    var aLi = box.children[0].children;
    for(var i=0;i<aLi.length;i++){
      aLi[i].style.backgroundImage = 'url(' + 'images/' + (i + 1) + '.jpg';
      aLi[i].onmouseover = function(){
        for(var i=0;i<aLi.length;i++){
          animate(aLi[i],{width:100});
        }
        animate(this,{width:800});
      };
      aLi[i].onmouseout = function(){
        for(var i=0;i<aLi.length;i++){
          animate(aLi[i],{width:240});
        }
      }
    }
  }