This paper studies the M_S5.8 earthquake that occurred in Alxa Zuoqi 2015. Based on the background seismicity and statistical analysis of the sequence, the HypoDD method was used to reposition the earthquake sequence. We examined the aftershocks distribution characteristics in space, and determined the M_S5.8 earthquake fracture surface combined with epicentral intensity、focal mechanism solutions, and the results of the re-positioning. The results show the Alxa Zuoqi MS5.8 earthquake is the main aftershock, after relocation, the epicenter is 39.78°N(±0.72 km)、106.34°E(±0.76 km), focal depth is 13.2 km(±1.15 km). The distribution direction of the sequence dominance is near NS and NEE, and extends 18 km and 16 km respectively. The focal depth of the profiles display the overall east-dipping sequence; the focal depth gradually deepened from west to east. The regional stress field background characteristics、historical earthquake rupture characteristics、the main shock mechanism solutions, and serial repositioning information considered judgment, the fracture surface of Alxa Zuoqi M_S5.8 earthquake should be near NS-direction surface.