<div class='zhezhao'>
<div>这是内容</div>
</div>
.zhezhao{
position:fixed;
top:0;
left:0;
bottom:0;
right:0;
background-color:rgba(0,0,0,0.5)
}
<div class='zhezhao'>
<div>这是内容</div>
</div>
.zhezhao{
position:fixed;
top:0;
left:0;
bottom:0;
right:0;
}