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I've recently changed my citizenship from Chinese to Canadian and I'm planning a short visit to China soon.

Is it possible to enter China with my existing Chinese passport (not expired yet) and then return to Canada with my new Canadian passport.

This becomes complicated since China does not recognize dual-citizenship. So do I need to get a China visa? Any input would be appreciated!

No, this would be illegal. Voluntarily naturalizing in another country results in automatic loss of Chinese citizenship. You need to contact your local Chinese embassy and obtain the proper visa.

The question mentioned by Traveller provides an overview of how you might or might not get caught trying to sneak back into China with an invalid passport. Don't risk it unless you enjoy the thought of doing time in a Chinese prison, and then being deported or removed.

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.

There was an article about a year ago in the South China Morning Post regarding people who attempt to benefit from Chinese citizenship when they are no longer entitled.

The government says they will confiscate the documents and deny entry. They've also clawed back some benefits that people have taken advantage of, including seizing a house (the house was provided in compensation for a relocation that foreigners were not entitled to). Other benefits such as schooling at local schools will also be denied, and use of the Chinese health care system as a citizen, tax breaks, easier ability to buy property and so on.

They've noticeably tightened up on information-gathering from entrants over the past couple of years and now collect fingerprints from foreigners at every entry and ask a great number of questions to qualify for a visa. If they have any competence in aggregating and correlating all this data with relatives and so on who have moved abroad they may know more than you think. Apparently they've cancelled the documents of more than a million people in a single year.

nope, that would be illegal, there's no treaty between china and canada about dual citizenship and even if there is, once you become a new citizen of another country you are revoking your old citizenship then you must reapply for it only there is agreement between the two countries, unfortunately for china they do not have such thing.