On November 23, 2015, an M_S5.2 earthquake occurred at Qilian county, Qinghai province. By the end of the first day of December, 48 aftershocks had occurred in the mainshock seismic region, including 34 M_L1.0~1.9 aftershocks, ten M_L2.0~2.9 aftershocks, and four M_L3.0~3.9 aftershocks, with the greatest magnitude being M_L3.4. In this paper, based on the seismic network data of Gansu and Qinghai provinces, we use the cut-and-paste method to calculate the focal mechanism solution of the Qilian M_S5.2. We then use the waveform fitting method for synthetic and observed seismograms to further determine the focal depth of the earthquake. The results indicate that this event occurred on a rupture plane (strike 109°, dip 58°, and slip 21°), which demonstrated a left-lateral strike-slip mechanism with a minor reverse-faulting component, a moment magnitude of M_W5.16, and a centroid depth of about 9 km. The other nodal plane was characterized by a strike of 8°, dip of 72°, and a slip of 146°. The focal mechanism, which describes the nature and source of the rupture process, provides an important foundation for understanding the stress state of the source region and the earthquake faults, and for analyzing the causes of a seismic event. A comparison of the maps of focal mechanism solutions for the Qilian M_S5.2 earthquake and historical earthquakes illustrates that the focal mechanism of this earthquake is in keeping with historical focal mechanisms. Therefore, based on our comprehensive analysis, we can identify the middle of the north Tuoleshan active fault as the seismogenic structure of this event.