"taxes are paid by the airline and should be also paid by your agency as stated in your / my contract."
Your contract will specifically state all CHINESE taxes due will be paid by the airline for you. YOU are responsible for any foreign tax due on your income. Many foreign countries have no ExPat tax but Canada and the US do tax your foreign earned income. As always consult a professional tax accountant, don't rely on pilots for tax advice or you will surely be audited.
The appropriate reference in this case is the Canada-China Income Tax Agreement 1986. Article 15 would be of particular interest and results in tax obligations that are very different than those that exist for US citizens. Using a tax accountant conversant with international tax treaties is an essential in this business.
ELAC
Agreement Between the Government of Canada and the Government of the People's Republic of China