基本上你不能：How to style the parent element when hovering a child element?
但技巧是使用兄弟元素：http://jsfiddle.net/k3Zdt/8/

.parent {
  width: 100px;
  height: 100px;
  padding: 50px;
}

.child {
  height: 100px;
  width: 100px;
  background: #355E95;
  transition: background-color 1s;
  position: relative;
  top: -200px;
}

.child:hover {
  background: #000;
}

.sibling {
  position: relative;
  width: 100px;
  height: 100px;
  padding: 50px;
  top: -50px;
  left: -50px;
  background: #3D6AA2;
  transition: background-color 1s;    
}

.sibling:hover {
  background: #FFF;
}

<div class="parent">
    <div class="sibling"></div>
    <div class="child"></div>
</div>

你可以欺骗一些东西;）
基本上，对子div使用:before伪元素，大小相同;
当鼠标悬停在子div上时，放大:before伪元素以覆盖父div区域;这将导致父div hover效果下降，然后返回到原始状态。还涉及z索引的精确组合。
Demo：http://jsfiddle.net/gFu8h/ Dark Magic（tm）

.parent {
    width: 100px;
    height: 100px;
    padding: 50px;
    transition: background-color 1s;
    background: #3D6AA2;    
    position: relative;
    z-index: 1;
}

.parent:hover{
    background: #FFF;    
}

.child {
    height: 100px;
    width: 100px;
    background: #355E95;
    transition: background-color 1s;
    position: relative;
}

.child:hover {    
    background: #000;
}

.child:before{
    content: '';
    position: absolute;
    top: 0;
    bottom: 0;
    right: 0;
    left: 0;        
    z-index: -1;
    transition: background-color 1s;
}

.child:hover:before{
    top: -50px;
    bottom: -50px;
    left: -50px;
    right: -50px;     
    background: #3D6AA2;    
}

<div class="parent">
    <div class="child"></div>
</div>