方法一：position加margin

<div class="wrap">
    <div class="center"></div>
</div>

.wrap {
    width: 200px;
    height: 200px;
    background: yellow;
    position: relative;
}
.wrap .center {
    width: 100px;
    height: 100px;
    background: green;
    margin: auto;
    position: absolute;
    left: 0;
    right: 0;
    top: 0;
    bottom: 0;
}

方法二： diaplay:table-cell

<div class="wrap">
     <div class="center"></div>
</div>

.wrap{
    width: 200px;
    height: 200px;
    background: yellow;
    display: table-cell;
    vertical-align: middle;
    text-align: center;
}
.center{
    display: inline-block;
    vertical-align: middle;
    width: 100px;
    height: 100px;
    background: green;
}

方法三：position加 transform

<div class="wrap">
    <div class="center"></div>
</div>

.wrap {
    position: relative;
    background: yellow;
    width: 200px;
    height: 200px;}
 
.center {
    position: absolute;
    background: green;
    top:50%;
    left:50%;
    -webkit-transform:translate(-50%,-50%);
    transform:translate(-50%,-50%);
    width: 100px;
    height: 100px;
}

方法四：flex;align-items: center;justify-content: center

<div class="wrap">
    <div class="center"></div>
</div>

.wrap {
    background: yellow;
    width: 200px;
    height: 200px;
    display: flex; 
    align-items: center; 
    justify-content: center;
}

.center {
    background: green;
    width: 100px;
    height: 100px;
}

方法五：display:flex;margin:auto

<div class="wrap">
    <div class="center"></div>
</div>

.wrap {
    background: yellow;
    width: 200px;
    height: 200px;
    display: flex; 
}

.center {
    background: green;
    width: 100px;
    height: 100px;
    margin: auto;
}

方法六：纯position

<div class="wrap">
    <div class="center"></div>
</div>

.wrap {
    background: yellow;
    width: 200px;
    height: 200px;
    position: relative;
}
/**方法一**/
.center {
    background: green;
    position: absolute;
    width: 100px;
    height: 100px;
    left: 50px;
    top: 50px; 
　　
}
/**方法二**/
.center {
    background: green;
    position: absolute;
    width: 100px;
    height: 100px;
    left: 50%;
    top: 50%;
　　margin-left:-50px;
　　margin-top:-50px;
}

方法七：兼容低版本浏览器，不固定宽高

<div class="table">
    <div class="tableCell">
        <div class="content">不固定宽高，自适应</div>
    </div>
</div>

.table {
    height: 200px;/*高度值不能少*/
    width: 200px;/*宽度值不能少*/
    display: table;
    position: relative;
    float:left;
    background: yellow;
}      
 
.tableCell {
    display: table-cell;
    vertical-align: middle;
    text-align: center;        
    *position: absolute;
    padding: 10px;
    *top: 50%;
    *left: 50%;
}
.content {
    *position:relative;
    *top: -50%;
    *left: -50%;
     background: green;
}

汇总

如果是移动端，那么用方法四和方法五是比较方便的。而且支持不固定宽高的情况，快、准、狠，也就是用 flex; align-items: center; justify-content: center; 

<!-- html -->
<div class="wrap">
    <div class="center"></div>
</div>
 
/* css */
.wrap {
    background: yellow;
    width: 200px;
    height: 200px;
    display: flex;
    align-items: center;
    justify-content: center;
}
 
.center {
    background: green;
    width: 100px;
    height: 100px;
}

或者  display:flex;margin:auto;

<!-- html -->
<div class="wrap">
    <div class="center"></div>
</div>
 
/* css */
.wrap {
    background: yellow;
    width: 200px;
    height: 200px;
    display: flex;
}
 
.center {
    background: green;
    width: 100px;
    height: 100px;
    margin: auto;
}

如果是PC端，要考虑兼容性的话。方法六是不错滴，也就是纯position。

<!-- html -->
<div class="wrap">
    <div class="center"></div>
</div>

/* css */
.wrap {
    background: yellow;
    width: 200px;
    height: 200px;
    position: relative;
}
/**方法一**/
.center {
    background: green;
    position: absolute;
    width: 100px;
    height: 100px;
    left: 50px;
    top: 50px;  
　　
}
/**方法二**/
.center {
    background: green;
    position: absolute;
    width: 100px;
    height: 100px;
    left: 50%;
    top: 50%; 
　　margin-left:-50px; 
　　margin-top:-50px;
}