By improving the critical slip field of slope, the numerical simulation method of bank collapse is proposed, considering lateral erosion and bed degradation. To avoid the unrealistic estimation of pore water pressure distribution, the method proposed here is to couple riverbank stability with groundwater flow simulation, and apply this to deal with the conjunction effect of river stage and groundwater flow. Numerical simulations of clay and silt bank collapse with representative parameters are carried out. With continual lateral erosion and bed degradation, stability of riverbank has dynamic changes. Modes of bank collapse of different soil riverbanks are also discussed. The results show that steep clay riverbanks fail along an almost planar failure surface which passes through the toe of the bank; and a gentle silt riverbank, exhibits a curved failure surface for which may occur partial collapse during rapid drawdown of river water level.