Abstract: Overpressure in sedimentary basins can be summarized into 3 types: disequilibrium compaction function,tectonic extrusion and liquid function.The forming mechanism includes undercompaction,montmorillonite dehydration,hydrocarbon generation,hydrothermal pressurization and hydrocarbon cracking,etc.But in deep formations,overpressure is mainly related to hydrothermal pressurization,superjacent strata pressure,hydrocarbon generation and cracking.So,overpressure coefficient in deep formation can be transformed by adding hydrothermal pressurization coefficient,superjacent strata coefficient and hydrocarbon pressurization coefficient.To estimate overpressure developed area,faults should be taken into consideration.Overpressure of the Lower Cambrian Niutitang Formation at the boundary among Hunan,Guizhou,Hubei and Chongqing has been studied.The most favorable region for overpressure locates around Yibin.