Abstract: Objectives:On December 18, 2023, Beijing time, an Ms6.2 earthquake occurred in Jishishan County, Linxia Prefecture, Gansu Province. This event occurred on the Lajishan Fault, a secondary fault with low activity and infrequent seismicity in the Qilian Mountains. This event provides a good opportunity to study this special structure within the Qilian Mountains. Methods:In this paper, we derived the co-seismic deformation field using both ascending and descending Sentinel-1A data. We applied the Bayesian inversion method to determine the source parameters of the seismogenic fault, and combined seismic wave records with InSAR data to model the dynamic slip distribution of this event. Results: The event has a northwest-dipping fault with a dip of 32.2°， a strike of 325.2° and a rake of 112°， indicating that this earthquake is a thrust event with a dextral slip component. Conclusions:By analyzing the fault geometry and kinematic characteristics of the primary active faults in the surrounding region, we found that the Jishishan earthquake occurred on the southern edge of the Laji Mountain. This study concludes that the Jishishan earthquake occurred on a northeast-dipping fault at the southern margin of the Laji Mountains. It also suggests that the eastern section of the Laji Mountain Fault, where the Jishishan earthquake occurred, may be a step-over structure between the Qinghai Nanshan Fault, the western section of the Laji Mountain Fault, and the West Qinling Fault. Additionally, it proposes that the left-running, right-step-type extrusion of the step-over zone is the driving mechanism of this retrograde seismic event.