Factors including river flow, geometry and rock-soil properties of river bank and cracks at the top of the slope are crucial for the stability of river bank slope. A generalized theoretical model of river bank slope instability under the influences of water level fluctuation, stream erosion at the slope toe, vehicle load and accidental action is established. The model is employed to study the influence of water level fluctuation, stream erosion at the slope toe and hydrostatic pressure on the slope stability of a river bank with polygonal line. With the limit equilibrium method, the slope stability factors under various influences are calculated. Calculation shows that the slope stability factor decreases and then increases with the rise of water level, giving rise to a most unfavorable water level in this rising process. At the same time, the slope stability factor reduces gradually when the fissure water level increases. Moreover, the stability factor declines with the increase of the erosion height and the cutting depth at the slope toe, demonstrating that stream erosion at the slope toe is detrimental to the slope stability.