$(window).scroll(function(){
     //执行处理的代码
})

如上所说：

$(document).ready(function(){
          var fixNav = function() {
            var Height = $(".top").height();
            var top = document.body.scrollTop;
            //console.log(Height);
            //console.log(top);
            if(top > Height){
               $(".left").css({"position":"fixed","top":"0"});
            }else{
               $(".left").css({"position":"relative","top":"0"});
            }
          }
            $(window).scroll(function(){
                fixNav();
            })
        });