Dynamic change process of stress field of north China before and after the Tangshan M_S7.8 earthquake is inverted by using the method inverting stress change of deep layer of the crust by reference[2] and data of well water level.Dynamic change characteristics of the stress field are analyzed.The results show anomalous region with stress decrease was produced in epicenter region and its vicinity two years before the earthquake. The anomalous region expanded outward and anomalous amplitude increased before impending the earthquake.The anomalous region contracted and disappeared gradually after the earthquake.