希望这可以帮助：
#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>
