As Robert Columbia mentioned, voluntarily naturalizing in a foreign country results in automatic loss of Chinese (PRC) citizenship (but not ROC nationality; I'll assume you are talking about the PRC), assuming you are not a resident of Hong Kong or Macau. Since you are no longer a Chinese citizen, you can no longer use your Chinese passport as proof of Chinese citizenship to enter China, even if the passport is unexpired. You must get a Chinese visa to visit China, and usually when you apply for your first Chinese visa, the Chinese consulate will ask for and cancel your Chinese passport.
If you still use your Chinese passport to enter China, you are pretending to be a Chinese citizen when you aren't. Although it probably won't be noticed when you go into China, you will have trouble leaving China. You said you will "return to Canada with my new Canadian passport", but China also has exit checks, so what passport will you use at Chinese exit checks? If you use your Canadian passport, it won't match any entry records since you did not enter China on that passport. If you use your Chinese passport, the Chinese exit checks will ask what document you will enter Canada with, since a Chinese passport is not visa-exempt to Canada. Presumably the only document you can show that will allow you to enter Canada is your Canadian passport, and if you show that, they will investigate how you acquired Canadian citizenship and whether you had already lost Chinese citizenship.