水平居中、垂直居中，总有一款适合你的

 .parDiv {
        position: relative;
        text-align: center;
        width: 100%;
        height: 300px;
        line-height: 300px;
        background-color: green;
    }
    
    .chiDiv {}

 .parDiv {
        position: relative;
        height: 300px;
        width: 100%;
        background-color: green;
    }
    
    .chiDiv {
        position: absolute;
        width: 100px;
        height: 100px;
        top: 50%;
        left: 50%;
        margin-top: -50px;
        margin-left: -50px;
        background-color: red;
    }

  .parDiv {
        position: relative;
        height: 300px;
        width: 100%;
        background-color: green;
    }
    
    .chiDiv {
        position: absolute;
        top: 50%;
        left: 50%;
        background-color: red;
        transform: translateX(-50%) translateY(-50%);
    }

 .parDiv {
        width: 100%;
        height: 300px;
        display: flex;
        background-color: green;
    }
    
    .chiDiv {
        background-color: red;
        margin: auto;
    }

      .parDiv{
           width: 400px;
           height: 400px;
           line-height: 400px;
           text-align: center;
           background-color: green;
       }
       .chiDiv{
           display: inline-block;
           line-height: normal;
           background-color: red;
       }

 <div class="parDiv">
        <div class="chiDiv">子容器内容</div>
  </div>