Abstract: Overpressure is generally developed in deep layer in Pinghu structural belts of Xihu depression, East China Sea Shelf Basin. Distribution characteristics, dominant factors of overpressure formation and their effects on the reservoir formation are systematicaly studied in this paper. The result shows that overpressure is generally developed in reservoir and mudstone of deep layer in the Pinghu structural belts, and the dominant factors of overpressure formation are different in related subunits; overpressure has no obvious effect on the thermal evolution of organic matter, but has contributed to the physical property of reservoirs; overpressure firstly accelerated hydrocarbon remigration in the early stage, but sealed off hydrocarbon to accumulate in the original place in the late period; overpressure enhanced sealing ability of deep mud cap rocks.