//html部分
<div class="main_box">
            <div class="box"></div>
</div>

第一種：

<style type="text/css">
        .main_box {
            height: 50rem;
            width: 50rem;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            margin:0 auto;
            position: relative;
        }

        .box {
            height: 100px;
            width: 100px;
            box
 
-sizing: border-box;
            border:1px solid #00FFFF;
            position: absolute;
            left: 0;
            right: 0;
            top: 0;
            bottom: 0;
            margin: auto;
        }
    </style>

第二種：

<style type="text/css">
        .main_box {
            height: 50rem;
            width: 50rem;
            box
 
-sizing: border-box;
            border:1px solid #00FFFF;
            margin:0 auto;
            position: relative;
        }

        .box {
            height: 100px;
            width: 100px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            position: absolute;
            left: 
 
50%;
            top: 50%;
            margin-top: -50px;
            margin-left: -50px;
        }
    </style>

第三種：

<style type="text/css">
        .main_box {
            height: 500px;
            width: 500px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            margin:0 auto;
            position: relative;
        }

        .box {
            height: 100px;
            width: 100px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            position: absolute;
            left: 250px;
            top: 250px;
            margin-top: -50px;
            margin-left: -50px;
        }
    </style>

第四種：

<style type="text/css">
        .main_box {
            height: 500px;
            width: 500px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            margin:0 auto;
            position: relative;
        }

        .box {
            height: 100px;
            width: 100px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            position: absolute;
            left: 250px;
            top: 250px;
            transform: translate(-50%,-50%);
        }
    </style>

第五種：

<style type="text/css">
        .main_box {
            height: 500px;
            width: 500px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            margin:0 auto;
            position: relative;
        }

        .box {
            height: 100px;
            width: 100px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            position: absolute;
            left: 250px;
            top: 250px;
            transform: translate(-50px,-50px);
        }
    </style>

第六種：

<style type="text/css">
        .main_box {
            height: 500px;
            width: 500px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            margin:0 auto;
            position: relative;
        }

        .box {
            height: 100px;
            width: 100px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            position: absolute;
            left: 50%;
            top: 50%;
            transform: translate(-50px,-50px);
        }
    </style>

第七種：

<style type="text/css">
        .main_box {
            height: 500px;
            width: 500px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            margin:0 auto;
            position: relative;
        }

        .box {
            height: 100px;
            width: 100px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            position: absolute;
            left: 50%;
            top: 50%;
            transform: translate(-50%,-50%);
        }
    </style>

第八種：

<style type="text/css">
        .main_box {
            height: 500px;
            width: 500px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            margin:0 auto;
            display: flex;
            justify-content: center;
            align-items: center;
        }

        .box {
            height: 100px;
            width: 100px;
            box-sizing: border-box;
            border:1px solid #00FFFF;
            
        }
    </style>