The Oligocene Huagang Formation in the Huangyan area of the West Subsag, Xihu Sag(some one calls the Xihu Sag as Xihu Depression), East China Sea Basin, is riched in oil and gas resources, which is an important area for exploration and development in the East China Sea shelf basin. However, the mechanism of oil and gas accumulation is complex. In order to clarify the process of oil and gas accumulation of the Huagang Formation in the West Subsag, this paper conducts research on the source reservoir cap, reservoir formation stages, and reservoir formation process through thin section observation, scanning electron microscopy, fluid inclusions, microscopic thermometry, and other experimental analysis, combined with structural evolution and burial history. The results show that the sandstone reservoir of the Huagang Formation has good reservoir conditions, while the Eocene Pinghu Formation develops effective source rocks, and also has good trap and preservation conditions. The early developed faults extend deep into the source rock of the lower Pinghu Formation and can serve as good migration channels. The development of deep overpressure also provides a certain driving force for oil and gas migration. Therefore, a “vertical superimposed” reservoir formation model has been established, indicating that the deep low permeability to ultralow permeability tight lithologic structural composite oil and gas reservoirs in the Xihu Sag and the oil and gas reservoirs in the deep formation with abnormal pressure systems have good exploration prospects.