div透明但不导致内容div透明的解决方法

 将两个层分开不嵌套即可

#div1{width:300px; height:200px;  position:relative；  } 
. nav{ background:black;filter:alpha(opacity:30);opacity:0.3; height:100%;position:absolute;width:100%;top:0; z-index:1 } 
.box { width:300px; height:20px; background:blue; position:absolute; bottom; top:0;z-index:0; } 

<div id="div1"> 
    <div class="nav"></div> 
    <div class="box">这里是内容部分</div> 
</div>