一般说来，在定语从句中，先行词指人时，关系代词通常用who/that引导；而当先行词指物时，关系代词通常用that/which引导。而用which指人的情况出现甚少。但在下列几种情况下，当先行词指人时，定语从句中的关系代词用which,而不用who
现归纳如下：
一、当先行词强调人物的身份、地位时。
They talked like a foreigner which they hardly were.他们说起话来像个外国人，而他们根本就不是。
Bill pretended to be a cleaner which helped him to escape from prison.比尔装成清洁工，这可帮他逃出了监狱。
二、当先行词指婴幼儿、小孩时。
My elder sister had a baby which was a girl yesterday.我姐姐昨天生了个女孩。
He made a face to the child which was in the mother's arm.他对抱在母亲怀中的婴儿做了个鬼脸。
三、当先行词强调个人的职业特征而非强调人时。
He is a professor,which commands our great admiration.他是个教授，教授是让我们极为钦佩的。
After graduating from university, he became a lawyer,which his relations wanted him to be.大学毕业后，他当了律师，这也是亲属们想要他从事的职业。
四、当先行词强调个人的性格特征时。
He is the man which he was.他还是象过去那个老样子。
Bonny is quite different from the girl which I thought her to be.邦妮完全不是我曾认为的那种女孩。
五、当先行词是集体名词被看作一个整体时。
Yao Ming is very interested in American public which warmly supports him.姚明对热情支持他的美国民众很感兴趣。
I take great pride in the Western Chinese people which has a long history, a rich culture and glorious revolutionary tradition.我作为西部的中华民族感到骄傲自豪，这是个有悠久历史、灿烂文化和光荣革命传统的民族。
六、当先行词强调人的品德素养时。
Sun Nan is a successful singer which gets him high fame in China.孙楠是个成功的歌手，这使他在中国名声大震。
He is a gentleman which his little brother isn't. 他很有绅士风度，他弟弟却没有。
参考资料：http://school.ecp.com.cn/school039/gzjs04-10/ca7549.htm