I kind remember that there is a rule: that the expiration date can not be longer than the expiration date of your passport.
Which means you want to renew your passport first. I heard that if you talk to the people in the Chinese Consulate nicely and tell them that you will renew study permit afterwards and you need a new passport to get the full 3 years, they will let you renew it even before the 6 months limit.
