Hi, I'm currently staying in Dongguan China. Can I extend my tourist visa even I'm in China?

Yes, Jason, you can do it.

What kind of documents I need to prepare? And where can I go to apply for extension?

Jason, you need to prepare passport, a fully completed application form affixed with a passport-style photo, valid registration form of temporary residence, the return ticket and accommodation arrangement.
You should apply for the extension at the exit and entry administration of the local PSB.