方案一：

盒子有设置宽高，直接margin: auto;

方案二：用position，left、right、top、bottom加margin调整

<style>
    .a{
        position: relative;
        width: 600px;
        height: 600px;
        border: 1px solid black;
    }

    .box{
        position: absolute;
        width: 200px;
        height: 200px;
        top: 0;
        left: 0;
        right: 0;
        bottom: 0;
        background-color: pink;
        margin: auto;

    }
</style>

<div class="a">
    <div class="box"></div>
</div>

方案三：position + transform：translate，水平垂直方向都可居中，例如

<style>

    body{
        /* position: relative; */
        margin: 0;

    }
    .a{
        position: relative;
        width: 600px;
        height: 600px;
        border: 1px solid black;
    }

    .box{
        position: absolute;
        width: 200px;
        height: 200px;
        top: 50%; /*盒子左上角的那个点在页面正中间*/
        left: 50%;
        background-color: pink;
        transform: translateX(-50%) translateY(-50%);/*不需要知道盒子宽度*/
    }
</style>

<div class="a">
    <div class="box"></div>
</div>

方案四：用弹性盒模型flex

<style>

    body{
        /* position: relative; */
        margin: 0;

    }
    .a{
        display: flex;
        justify-content: center; /*水平居中*/
        align-items: center; /*垂直居中*/
        height: 600px;
        width: 600px;
        border: 1px solid black;
    }

    .box{

        width: 200px;
        height: 200px;
        background-color: pink;

    }
</style>

    <div class="a">

        <div class="box">

        </div>
    </div>