Iran has few friends, and a lot of oil (even though they can't refine it).
PR China is desperately searching for reliable, for them, sources of energy. I read, last week, in China Daily News, an article by a Chinese Gov. official (I think), that was proposing that China build two refineries in Iran.
Iran would gain some, much needed, energy, for it's industry and transportation, until it's nuclear power reactors are running. PR China would gain access to a reliable, and huge, source for crude oil.

It's a win/win situation for both countries, besides Iran gaining a powerful supporter in the UN, with veto rights.