仅需要加上：margin: auto;
html:
<body>
<div id="box">
<div id="header_left"></div>
</div>
</body>
css:
#box{
width: 100%;
height: 300px;
background-color: #333333;
text-align: center;
}
#header_left{
width: 70%;
height: 200px;
background-color: #222222;
margin: auto auto;
}
