<div class="parent">
    <div class="left">我是左边固定</div>
    <div class="right">我是右边自适应</div>
</div>

.parent{
    width:100%;
    height:400px;
}
.left{
    float:left;
    width:200px;
    height:100%;
    background:#afa;
}
.right{
    height:100%;
    margin-left:200px;
    background:yellow;
}

.parent{
    position:relative;
    width:100%;
    height:400px;
}
.left{
    position:absolute;
    left:0;
    width:200px;
    height:100%;
    background:#afa;
}
.right{
    margin-left:200px;
    height:100%;
    background:#aba;
}

.parent{
    width:100%;
    height:400px;
    display:flex;
}
.left{
    width:200px;
    background:#afa;
}
.right{
    flex:1;
    background:yellow;
}