1.which引导的定语从句只能位于主句后,但as引导的从句可位于主句前、中、后.如：
David,as you know,is a photograopher.戴维是个摄影师,你是知道的.(不可用which)
Li Ming is late,as is often the case.李明迟到了,这事是经常发生的.(不可用which)
2.as在定语从句中作主语时,从句谓语通常要有be或别的系动词,但which不受此限制.如：
Li Ming was late,which (=and this) made Mr.Zhang very angry.李明迟到了,这件事使张先生非常生气.(不可用as)
3.as引导的定语从句只表示一个众所周知或意料之中的事,但which不受此限制.如：He married her,which was unexpected.他和她结婚了,这是出乎意料的.(不可用as)
4.as有“正如”之意,而which则意为“这(件事,一点)”,可用and this/that/it来代替.
5.as多用于固定搭配中：as is often the case(这是常有的事),as was expected(不出所料),as often happens(正如经常发生的那样),as is known to all(众所周知),as has been said before(如上所述),as is mentioned above(正如上面所提到的)等.
注：which代表整个句子,还可用于in which case,at which point,on which occasion等.如：
I may have to work late,in which case I’ll telephone you.我可能得晚点下班,那样我会给你打电话的.