<div id="flex">
  <div id="left">我在左邊，自適應布局</div>
  <div id="right">我在右邊，定寬</div>
</div>

#flex{
  display: flex;
}

#left{
  flex:1 1 auto;
  background: red;
}

#right{
  background:green;
  width: 200px;
}

方案二：

#left {
   position: absolute;
   width: 100px;
   float: left;
}

#right {
  margin-left: 100px;
}