You have more problems than you think... First, you're not, legally, a PRC citizen anymore: by acquiring Canadian citizenship, you forfeited your PRC citizenship. It's not that the PRC doesn't like dual citizenship, it doesn't allow it... And if you are currently in China on your PRC passport, you are indeed in trouble (should you get caught).
Showing your PRC passport to the airline staff won't get you on the plane: there's no US visa on it. Showing only your Canadian passport won't get you on the plane either, as there's no China entry stamp: you'd have to show also your PRC passport. It's possible that the airline staff won't mind, but there's no guarantee that they won't notify Immigration.
However when you show up at the Immigration counter, and hand over your PRC passport and boarding pass to the officer, there won't be a US visa either. And that won't fly with them: they are just as thorough with PRC citizens leaving the country as with foreigners entering the country. You won't be let through, most probably – and will probably be escorted to secondary, and possibly more than that. You'll lose your PRC, and be sent on your way, hopefully straight away.
My first piece of advice is: leave China via a visa-free country (Malaysia just opened its doors to PRC citizens), or, it might work, via HK or Taiwan. From there you can fly to the US on your Canadian passport.
My second piece of advice would be to renounce officially your PRC citizenship – it will save you more trouble in the future, and as a Canadian citizen you can get a 10-year visa anyway. Plus, you won't be able to extend/renew your PRC passport in Canada or the US, as there's no Canada / US visa on it. That would cancel your citizenship too...
As @jcaron mentions I misread the question, and the US visa is in the PRC passport, so this gives you much more leeway – unless the airline staff passes on to Immigration the info that you are a dual citizen, you should be able to leave the PRC unscathed. And with a visa on your PRC passport, you should be able to renew it in the US (but not in Canada).