Using the Lanzhou telemetered seismic net digital data, studies the geometric parameters of the sources of Yongdeng aftershocks in Gansu Province and discusses the source mechanism and the 3-D distribution pattern of aftershocks sources. The aftershocks of the Yongdeng earthquake are distributed on two crossed fault surfaces and their origin time has obvious group characters. The epicenter of main shock is just at the point where the two faults cross. The depths of two faults are also different. It is convinced that the source process of the Yongdeng M_S 5.8 earthquake belongs to the interlayer-gliding type.