Abstract: There commonly exists overpressure in shale gas reservoirs in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formations in marine strata in South China. Some typical wells in the Fuling Shale Gas Field were chosen to study systematically to reveal how the overpressure does on shale gas reservoirs. Overpressure is the direct shows of favorable preservation conditions for shale gas. Pressure increases when hydrocarbon generates from organic matter in thermal evolution, which is the main cause for abnormal high pressure in the region. Overpressure exists in all high yield intervals for shale gas in the Longmaxi Formation in Jiaoshiba of Fuling area. The shale reservoirs have high porosity and high gas content. The effect of overpressure on shale gas reservoirs was concluded. On the one hand, overpressure can slow down the effective stress on rock skeleton, so that the shale reservoirs preserve a relatively higher porosity, and overpressure can form shale overpressure micro-cracks, improving the seepage capacity of shale reservoirs. Overpressure indicates for high formation energy, high free gas content, and better gas content in shale gas reservoirs. On the other hand, overpressure reduces the effective stress on shale reservoirs, and the development of overpressure micro-cracks is conducive to shale fracturing.