Edit in JSFiddle

<div class="wrap">
    <div class="content clearfix">
        <div class="main">中-等高吗？
            <br />中-等高吗？
            <br />中-等高吗？
            <br />中-等高吗？
            <br />中-等高吗？
            <br />
        </div>
        <div class="side">左-等高吗？
            <br />
        </div>
    </div>
</div>

html, body {
    margin:0;
    padding: 0;
}
.clearfix {
    zoom: 1
}
.clearfix:after {
    content:"";
    display: block;
    height: 0;
    line-height: 0;
    font-size: 0;
    visibility: hidden;
    clear: both;
}
.wrap {
    width: 950px;
    margin: 0 auto;
    background: #ccc;
}
.content {
    margin-left: 150px;
    border-left: 1px solid #666;
    background: #ddd;
}
.main {
    float: left;
    width: 100%;
}
.side {
    float: left;
    width: 150px;
    margin-left: -950px;
    position: relative;
}