function getInput(val,a){

    var id = 'ser-key';

    if(a=='focus'){

        document.getElementById(id).value='';

    }

    if(a=='blur'){

        if(val==''){

            document.getElementById(id).value= "请输入VIP卡号或手机号码查询";

        }else{

            document.getElementById(id).value= val;

        }

    }

}

 <input name="keyword" type="text" class="inp_a" id="ser-key" onfocus="getInput(this.value,'focus')" onblur="getInput(this.value,'blur')" value="请输入VIP卡号或手机号码查询" size="" />