We calculated the focal depth of the Shizuishan earthquake by using arrival-time difference of seismic phase sPn and Pn,which is 7.21±0.277km. The depth of earthquake is 7~8km by comparing the synthetic seismograms and the observed seismogram at different depths. Two results show that the sPn phase can determine reliable source depth,sPn and Pn in the upper and lower crust of the border area between the upper and the lower crust and the source depth of the upper and lower crust are obtained. According to the regional velocity model,we obtained the corresponding table of arrival-time difference and focal depth when earthquakes occur in the upper and lower crust in the border area of Ningxia and Inner Mongolia. The Shizuishan earthquake occurred in the north boundary of the Yinchuan Basin. Shallow source is one of the important reasons for feeling strongly in source region.