Abstract: The deformation and instability of bank slope under the condition of water level rise and fall is very important for the safety of reservoir construction. The dual structure is a special structure of rock and soil mass on the bank of reservoir. Its deformation and failure characteristics have its special mechanical mechanism and law. In order to reveal the instability mechanism of bank slope under the condition of different slope angles and different soil rock interface angles, especially the condition of water level change, the distribution and evolution characteristics of the saturation line of the rock and soil mass in the lower reservoir bank are studied in this paper by constructing the physical experiment model of the dual structure reservoir bank slope under the condition of water level rise and fall, and by means of monitoring and photography the characteristics of the saturation line, and the deformation and failure of the rock and soil mass in the slope are observed. The deformation and instability mechanism of the dual structure reservoir bank slope are revealed. The research results show that under the condition of the dual structure reservoir bank slope during the water level rise and fall, the change in the overall slope angle will lead to different deformation and failure modes：the 55° slope is dominated by collapse and instability, the 35°slope has good stability, and the 45° slope forms local instability pulled by slope toe failure. The slope stability will be greatly affected by the inclination of soil rock interface, which is easy to cause sliding instability of slope along the soil rock interface. The results also show that the mechanism and effective prevention and control of the landslide can be used for reference.