It's well known by now that the Chinese account for more than 50% of the mining power of Bitcoins network. Is that reason to worry? In my opinion the answer is no. The biggest risk that the Chinese control over the Bitcoin network poses is the possibility of a 51% attack which would effectively kill BTC over night. However a 51% attack would require that one entity controlled the vast majority of Chinese hashing power and that isn't the case as of right now. There's also little reason for the Chinese government to do kill BTC, they've undoubtedly invested billions into mining Bitcoin and killing it would hurt them more than anyone else. 

BUT

That doesn't mean you shouldn't worry at all about the fact that China controls so much of the network, it's always possible that US lawmakers decide that Bitcoin is a threat to the US due to the simple fact that China controls so much of it and they may try to regulate it as a result.