Abstract: Extrapolating the geothermal analysis results of wells into the area without well on the corre-lativity between the temperature and the formation depth or the bottom of the lithosphere,this paper made the geothermal model of all the Xihu Sag in the East China Sea Basin,and analyzed its effect on the Pinghu Formation source rock.The result shows that the geothermal gradient in the northern and southern area is high.The maturity of the Pinghu Formation in the northern area is more than the one in the southern area.In despite of the reserves found in the southwest of the Xihu Sag,the explore potential of the northern area must be emphasized.Most of the hydrocarbon expelled from the Pinghu Formation during 30～10 Ma.The fact that the traps are large,but their hydrocarbon filling degree is little in the Xihu Sag,is the result of the absence of the preservation or the insufficient hydrocarbon charge.The study on the duration relationship between the trap formed and the hydrocarbon expelled is the key to find more reserves in the future in the Xihu Sag.