Abstract: Seismicity results from the rapid release of elastic energy during unstable fault slip. Recent studies have demonstrated that high-pressure fluid injection during hydraulic fracturing for reservoir stimulation can induce fault activation and seismic events. The stick-slip behavior associated with unstable fault slip plays a key role in triggering such seismicity. This study investigates the frictional behavior of fault instability induced by fluid injection,with comparative analysis of dry(non-fluid-injection) conditions. Through triaxial compression experiments on shale specimens,we examine fault slip instability under two loading conditions:(1)axial stress-driven slip and (2)constant-rate fluid injection. By monitoring stress and displacement evolution while calculating shear stress and friction coefficients,we systematically analyze how fluid injection affects post-instability frictional behavior. Key findings reveal:(1)Under dry conditions,the friction coefficient exhibits cyclic weakening and recovery around~0.6 following stick-slip events. (2)With fluid injection,immediate stress drop triggers instability,evolving into prolonged slow slip with high-frequency stick-slip recurrence. (3)Subsequent frictional weakening leads to significant slip displacement until fluid pressure approaches the minimum principal stress,when strengthening behavior dominates-ultimately ceasing slip as sliding velocity asymptotically approaches zero. The experiments demonstrate that fluid injection can reduce the friction coefficient from ~0.6 to near-zero values,unequivocally establishing pore pressure’s controlling influence on fault stability. These results provide critical insights for preventing induced seismicity during reservoir stimulation through optimized pore pressure management.