.box {
            height: 500px;
            display: flex;
        }
        .box div {
            height: 300px;
            text-align: center;
        }
        .box div.right {
            flex: 1;//右边自适应
            border: 1px solid #000000;
        }
        .box div.left {
            border: 1px solid #000000;
            flex-basis: 100px;//左边固定,优先级最高
        }
    //html
    <div class="box">
        <div class="left">左边固定效果</div>
        <div class="right">右边自适应效果</div>
    </div>