Abstract: The fluid overpressure is commonly found in the Pinghu Formation, Pinghu structural belt, Xihu Sag in the East China Sea. According to regional geology study results, drilling data,well-log analyzing results, related testing data the drilled wells in Pinghu structural belt have been plotted with Eaton method, and the distribution of the formation overpressure is revealed. In addition, the main reasons for formation overpressure have been analyzed, which are caused by undercompaction and hydrocarbon generation. The contribution of undercompaction and hydrocarbon generation to overpressure formation has been analyzed by loading and unloading procedure. The study results are significant for study HC accumulation and design of drilling engineering.