Boyfriend is a green card for investing in immigration to Canada. He wants to marry me and apply for my past. I have been married, divorced and remarried, but all by myself.

The government there won't care how many times you have been married or divorced, as long as you have legal procedures to marry him, you will accept it, so don't care about your past marriage history and submit a formal marriage certificate. To apply for a marriage certificate in China, you must go to a notary office for translation and notarization, and other matters are handled by the government.