I have been married to a Chinese citizen for 6 years. Can I apply for Chinese citizenship ?

If you have lived in China for five years in a row, the annual stay in China being no shorter than nine months, and have stable and secured living status and place to live in China, you may apply for a premanent residence permit for Spouse Reunion. Here is the information for your reference: https://www.travelchinaguide.com/embassy/visa/refusal.htm#spouse
If you don't meet these conditions, you can think about applying for a temporary resident permit. FYI: https://www.travelchinaguide.com/embassy/visa/family-visit-permit.htm