Edit in JSFiddle

<div class = 'father'>
  <div class = 'left'>左边左边</div>
  <div class = 'right'>
    右边右边<br>
    右边右边<br>
    右边右边<br>
    右边右边<br>
    右边右边<br>
  </div>
</div>

.left{width:200px;
  float:left;
  background:#FAD7F0;
}
.right{
  flex:1;
  background:lightblue;
}
.father{
  display:flex;
  flex-direction:row;
  align-items:stretch;
}