Abstract: In view of the gradual riverbank collapse, a hydrodynamic model for assessing riverbank stability is established on the base of soil mechanics and river dynamics theory. Combined with generalized model tests and numerical calculation, the mechanism of riverbank stability is analyzed, and the failure reason for slight slopes is explained. Results show that the slope should be stable when the seepage exit gradient on the slope surface is less than the critical gradient of infiltration failure. After the toe of the slope was washed away by current, partial infiltration failures on the slope begin to appear and will develop with the increase of the seepage exit gradient due to the shortening in seepage path length. The back soil mass will fail successively due to lack of support. The gradual soil mass failure will develop backward with time leading to the collapse and destruction of the entire slope eventually.