I am a Chinese, but I got my J1 exchange scholar visa to enter U. S from Canada (I have been living in canada for 6 years but am NOT an immigration in canada).

The tax people said that my tax would be based on the canadian policy. Is that true? Am I eligible to tax exemption?

Thank you.

The visa is stamped on the passport you are holding. If you are a Chinese citizen, you will get deduction as per the U.S. China tax treaty. If you are a research scholar, then your income is not subject to tax for three years.
Read this about the tax treaty: Your U.S. Tax Return: U.S. Tax Treaties for Professors, Teachers and Researchers