function li1(){

   document.getElementById("l1").style.display = "block"

   document.getElementById("l2").style.display = "none"

   document.getElementById("l3").style.display = "none"

}

<li id=l1 onmouseover="li1()">房产</li>   

<li id=l2 onmouseover="li2()">家居</li>          

<li id=l3 onmouseover="li3()">二手房</li>