Abstract: Objective On August 26, 2021, an Ms5.5 earthquake occurred in Aksai, Gansu Province.  The epicenter located along the southern piedmont of Danghe Nan Shan. This event has garnered significant attention regarding its deformation characteristics and seismogenic mechanisms. Existing studies mainly focus on emergency response and seismic activity analysis, but there is a lack of research on tectonic deformation and seismic mechanism. This study aims to fill this gap by analyzing the deformation characteristics of the earthquake zone and revealing its seismogenic mechanism. Methods This study employs seismological methods combined with InSAR (Interferometric Synthetic Aperture Radar) technology to investigate the tectonic deformation and seismic mechanism of the 2021 Aksai Ms5.5 earthquake. By combining focal mechanism solutions, precise earthquake location, and InSAR results, the seismogenic fault and its geometric and kinematic parameters are determined and validated through field geological surveys. Results This study applied the joint inversion with both local and teleseismic waveforms (gCAPjoint) to source parameters, the fault solutions are strike 315°, dip 41°, rake 81°, depth 6.9 km. We relocated Aksai earthquake and its aftershocks using the Hypoinverse and the double-difference method (HypoDD), and finally the accurate locations of 88 earthquakes were obtained. The 2021 Ms5.5 earthquake sequence in Aksai is distributed near the southern Danghe Nan Shan fault, with a fault dip toward the NE. The co-seismic deformation field indicated by InSAR matches the macro-epicenter with the precise location results, confirming the reliability of the precise location. Both ascending and descending orbit surface deformation fields show uplift near the epicenter, with similar magnitude and sign in the line-of-sight direction, indicating that the earthquake rupture was mainly thrusting. The fault scarps near the epicenter, along the southern piedmont of Danghe Nan Shan, are recognized in the field and satellite images. Combined data from focal mechanism solutions, precise earthquake location, and InSAR coseismic deformation fields, along with field geological survey results, indicate that the seismogenic fault of this event is the southern Danghe Nan Shan fault, with a strike of 315°, dip of 41°, and rake of 81°. Conclusion This study indicates that, the seismogenic fault of this event is the southern Danghe Nan Shan fault, it is a thrust fault. The fault solutions are strike 315°, dip 41°, rake 81°, depth 6.9 km. Due to the northward extrusion thrust of the Qinghai-Xizang block, seismic activity in the northern part of the Qaidam block has significantly increased.  The future seismic risk of the central and eastern sections of the Altyn Tagh fault and the western Qilian Shan needs to be emphasized. Signficance This study provides new insights and methods for the research of active tectonics. It holds significant scientific importance and innovation in understanding seismogenic mechanisms and structural transformation, as it helps to understand the mode and magnitude of slip transfer between the strike-slipping of the Altyn Tagh fault and the shortening of the Qilian Shan, and also contributes to better evaluate the seismic risk in this region.