The focal mechanism of M_S 5.4 earthquake on June 1, 1996, in the Tianzhu pull-apart basin of western Haiyuan fault is studied. The seismogenic fault of the event is determined by using the precisely locating aftershocks, which recorded by the local digital seismic network.The result shows that the earthquake caused by the fracture of a nearly SN direction small fault, which perpendicular to western Haiyuan main fault, in the Tianzhu basin. The seismic risk along western Haiyuan fault is discussed on the basis of the fracture model and the condition of stress concentration in the area.