I assume that you have no other citizenship except the US one, and China has already known that you are an american, not a chinese any more. When you go to a U.S. Consulate to ask for renunciation, you can do it. However, you will be warned that after you give up US citizenship, you will be a person has no nationality and no valid passport. So you should contact the Chinese Embassy first and ask for their law or a written promise that you can become a Chinese again. If I were you, I would not take that kind of risk.
