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Sorry, no Chnese input:)

Going back to China: At the flight check in counter, using your Canadian passport since your visa is on that; When entering China, also show them your Canadian passport with your visa on it;

Coming back to US: At the flight check in counter (US citizen has to enter US via his US passport), the staff might need take a look at your Canadian passport and the visa on it); Show China BIAN JIAN your Canadian passport since you entered China using your Canadian pssport; When arriving back US, show your US passport (BTW, you can download a Mobile passport app and use it when going through the customs, very short line and fast!)

I just came back from China and for your reference.