Point 1: State clearly that you are interested in pursuing a PhD in Econ and what you envision yourself doing with your Econ PhD (becoming a researcher should be the main goal).
Point 2: Elaborate why you are prepared from an academic point view in terms of classes. No need to list all of your classes, but stress that you have the proper math training.
Point 3: Talk about any research experience. Here, please do not go into too much detail on exactly what you did, but instead focus on the fact that you know what research is like, but also know that you still have a long way to go until you can conduct independent research. And a PhD will help you become a better researcher.
Point 4: Any other preparations you have done to prepare yourself and your field interest. Explain why.
Point 5: Why the school you are applying to is a good fit and if applicable, what you bring to the table. Why are you different from any other Chinese applicant. Conclude with the fact you would like to become a qualified researcher.