On September 20, 2013, an earthquake of M_S5.1 occurred in the Sunan-Menyuan boundary region. Using the data recorded by Gansu and Qinghai Digital Seismic Networks, we relocated the M_L ≥ 1.0 events in the earthquake sequence using the Hypo2000 location algorithm. After relocation, the vertical and horizontal average errors were found to be 1.87 km and 3.64 km, respectively, and the root mean square residual error of travel time is 0.51 s. The results show that the mainshock occurred at the northern foot of Gangshiga, the main peak of Lenglongling, and the aftershocks are primarily distributed along the Lenglongling fault zone, with a length and width of approximately 20 km and 2.5 km, respectively. The transverse section exhibits focal depth focus in 3~10 km, and longitudinal section shows a fracture plane that dips toward the northeast and the dip angle is approximately 30°. The focal mechanism solution of the mainshock is also determined using the cut and paste(CAP) method. The results show that the best double couple solutions of the event and the other nodal plane are 360°, 68°, and 117° and 126°, 34°, and 42°, respectively, for strike, dip, and rake angles. Further, the moment magnitude is MW5.05, and the best centroid depth is 10 km.