Abstract: Step-pool is a controlling river bed form which frequently develops in mountain streams. Sediment transport significantly affects the stability of step-pool system by dislodging the keystone with large grain collision. The keystone of an individual step-pool was selected as the object for collision force analysis and the grain impact was quantified to add into the theoretical model for individual step-pool stability, which was then utilized for the analysis of critical conditions and the failure mechanism. The grain impact is positively correlated with grain size, grain initial velocity and the downstream scour of the step. The critical discharge for step-pool failure largely decreases under collision and larger grain size leads to the smaller threshold. The critical discharge would experience a decrease of over 50% if η > 0.55 (η=D₁/D, where D₁ is the diameter of the impacting grain; D is the diameter of the keystone), indicating that the large grains in bedload have a significant influence on the stability of an individual step-pool. When a flood event with long return time, landslide or debris flow occurs, the collision between the large grains and the step would enlarge the driving torque and reduce the critical discharge, resulting in much easier access to the failure of an individual step pool. This model could effectively evaluate the step-pool stability precisely when applied to step-pool reaches.