区别：
1）as可指其前后主句提到的事实或情况，因此引导的非限制性从句位置比较灵活，which可以放在主句前后，也可插入主句之中；而which引导的非限制性从句只能位于主句之后：
As is known to all, fish can’t live without water.
Air, as we know, is gas.
2）在非限制性从句中，which后的be动词不可省；as而后的be动词可以省略：
She told me she won the match , which was a lie. The material is elastic, as (was) shown in the figure.
3） which在非限制性定语从句做主语时可用各类动词做谓语；而在非限制性定语从句做主语时，谓语常用连系动词，如be, seem, become等，一般不用其他行为动词：
He saw the girl, which delighted him.
He didn’t say anything at the meeting , as/which seemed very strange.
4） 如果主句中叙述的事情与从句所涉及的意向一致，多用as，用以对主句的意义进行补充；如果不一致，就用which：
She has married again, as was expected.
He broke the rules again, which was unexpected.
5）如表示“如同。。。那样，按照，正如”含义，常用as，这里as主要起与上下文联系的作用，表达说话人的看法，并指出内容，出处和根据等，常用的结构有as we know（众所周知）as often happens（正如常发生的那样）as is often the case（情况常常如此）as we all can see（正如我们看到的）
be+announced/expected/known/imagined/mentioned/said/shown/reported（正如所。。。）等。这些结构常放句首，偶尔也可以放句中或句末 ：
As is known to all, China is a developing country. Kate was late for school , as often happened.
6）在非限制性定语从句中关系代词指代主句中某一个单词时，常用which：
My brother enjoyed playing basketball , which he really plays well. Beijing , which he was born in, is our capital.
7）在非限制性定语从句中“介词+关系代词（介宾代物）”中关系代词只能用which ：
The Travel Agency, with which our company has been dealing for several
years, has opened for new branches
Air is a mixture of gases, of which oxgen forms 21 percent.
8）有时用case, point, time, situation或reason等抽象名词来总结主句的内容，可用“介词+ which +抽象名词”构成从句 ：
Ten years of hard work changed her greatly , for which reason he could hardly recognise her at first sight.
The machine may be out of order , in which case it will be repaired at once.
9）在非限制性定语从句关系代词用以代表主句中谓语的整个概念，从句中的谓语动词是否定形式时，常用which：
He can write a letter in English, which I can not.
Metal will bear beating with a hammer, which a stone willl not
10）在非限制性定语从句关系代词代表前面主句中的宾语从句时，常用which ：
He said he had passed the exam , which was untrue.
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