快点一个，看看吧。

<div id="left">Side menu</div>
<div id="right">Scroll
    <br />Scroll
    <br />Scroll
</div>

CSS：

CSS:

html, body {
    height: 100%;
    margin: 0;
    font-size: 20px;
}
#left {
    width: 20%;
    height: 100%;
    position: fixed;
    outline: 1px solid;
    background: red;
}
#right {
    width: 80%;
    height: auto;
    outline: 1px solid;
    position: absolute;
    right: 0;
    background: blue;
}

DEMO HERE