Do I need to cancel my account after I get an American immigrant visa?

No, if you only hold an American green card, you still use a China passport and are a citizen of China. In this case, there is no need to cancel your account.

However, if you apply for naturalization in the United States and become an American citizen, according to the provisions of the Nationality Law of China, after you acquire another nationality, it means that you automatically give up your China nationality and your account will be required to be cancelled.