代码如下,希望帮助到有需要的朋友:

#mainDiv {
  height: 100px;
  width: 80px;
  position: relative;
  border-bottom: 2px solid #f51c40;
  background: #3beadc;
}

#borderLeft {
  border-left: 2px solid #f51c40;
  position: absolute;
  top: 50%;
  bottom: 0;
}
<div id="mainDiv">
  <div id="borderLeft"></div>
</div>