我想你打算这样做(第二个使用 ajaxStop()):

I think you meant to do this (using ajaxStop() for the second one):

$(document).ajaxStart(function(){   $("body").addClass("loading"); });
$(document).ajaxStop(function(){   $("body").removeClass("loading"); });

查看它在jsFiddle的修改版本上的工作原理: http://jsfiddle.net/jfriend00/gk3RL/

See it working here on a modified version of your jsFiddle: http://jsfiddle.net/jfriend00/gk3RL/

此外，使用 document.body 代替"body" :

$(document).ajaxStart(function(){   $(document.body).addClass("loading"); });
$(document).ajaxStop(function(){   $(document.body).removeClass("loading"); });