Abstract: The seepage and erosion are the key factors for riverbank collapse. In this paper, based on the probability theory and basic principle of sediment movements, we introduce threshold probability and relative exposure degree and use the sliding-incipient model to deduce a unified formula of riverbank sediment incipient motion velocity in consideration of the seepage effect. The presented model makes up the deficiency of the exiting studies that do not consider the incipient probability and seepage force. Under simplified incipient boundary condition, the unified formula can be simplified into the incipient motion velocity equation adapting to the straight slope, positive and negative slope, horizontal ones and so on. As the verification shows, the calculated sediment velocity is well consistent with the measured data in two conditions of with and without seepage effect. The results are helpful to deeply understanding the sediment incipient motion and riverbank collapse mechanism.