In order to gain a Chinese citizenship, you have to FIRST gain your Foreigner's Certificate for Permanent Residence in the PRC (中华人民共和国外国人永久居留证, aka, Chinese green card). This is the most difficult part.
To be qualified for the certificate you must either:
- Hold a relatively high-level post in China
- Have direct and stable investment in China
- Have made some big/important contributions to China or be the kind of professionals that China needs desperately
- Be a spouse or unmarried child under 18 of the ones that meet at least one of the 3 above requirements
- Be a spouse of a Chinese citizen
- Be a unmarried child under 18 whose parents are Chinese citizens
- Be over 60 years old and have direct relatives in China and none outside China
And you must have no criminal records.
More importantly the Chinese govt does not issue much of these certificates every year. That is to say, even if you are qualified you could most probably still be denied such a certificate.
It's said that Hong Kong SAR citizenship is easier to get. Of that I have no knowledge.