Abstract: It's observed that oil/gas fields distribution is distinctively connected with the geothermal abnormal areas in continental margin basins of the northern South China Sea.This phenomona is basically due to the geothermal fluid vertical movement and the lower thermal conductivity of oil/gas beds during the process of oil/gas reservoir formation.Thermal evolution of sedimentary basin has significant impaction on the range of oil/gas window,and also the maturity of vitrinite reflectance can be restrained as a result of overpressure.Therefore it has been taken into consideration in this paper when the vitrinite reflectance is calculated with EasyRo model.The simulated result indicates that the fact of overpressure has expanded the range of oil/gas window and exploration scope as well.