After water impoundment in the Three Gorges Reservoir, the physical and mechanical properties of bank slope rock mass have inevitablely deteriorated under the influence of many factors such as dramatic water level fluctuation, rainfall, and low strength earthquake of high frequency, which directly affects the deformation stability of reservoir bank slope. In view of this, we analyzed and assessed the seismic behavior of a typical reservoir bank slope of Three Gorges Reservoir in consideration of the degradation of rock mass shear strength parameters. Results show that for the studied bank slope, 1) under different characteristic water levels, the global safety factor of slope increased slightly as water level raised, but reduced apparently as earthquake acceleration increased; 2) at all levels of earthquake action, the anti-seismic capacity of bank slope gradually reduced in the presence of degradation of shear strength parameters, with possible local instability in the mid-lower part of slope accumulation body; and as water level raised, the trailing edge of unstable region gradually raised. The research ideas and achievements offer reference for the stability evaluation and reinforcement of reservoir bank slope.