HTML

 

<div class="fater">
    <div class="sum"></div>
</div>

方式一: 父级相对 子级

　　要求子级宽度设置

<style>
    .fater{
        width: 500px;
        height: 500px;
        border: 1px solid black;
        position: relative;
    }
    .sum{
        width: 100px;
        height: 100px;
        background: red;
        position: absolute;
        left: 50%;
        top: 50%;
        margin-left: -50px;
        margin-top: -50px;
    }
</style>

方式二：transform:translate(-50%);

　　宽高不确定,transform是css3属性注意兼容性问题

<style>
    .fater{
        width: 500px;
        height: 500px;
        border: 1px solid black;
        position: relative;
    }
    .sum{
        width: 100px;
        height: 100px;
        background: red;
        position: absolute;
        left: 50%;
        top: 50%;
        transform: translate(-50%);
    }
</style>

方式三

　　子级宽高不确定

<style>
    .fater{
        width: 500px;
        height: 500px;
        border: 1px solid black;
        position: relative;
    }
    .sum{
        width: 100px;
        height: 100px;
        background: red;
        position: absolute;
        left: 0;
        top: 0;
        right: 0;
        bottom: 0;
        margin: auto;
    }
</style>

方式四:flex布局

<style>
    .fater{
        width: 500px;
        height: 500px;
        border: 1px solid black;
        display: flex;
        align-items: center;
        justify-content: center;
    }
    .sum{
        width: 100px;
        height: 100px;
        background: red;
    }
</style>