The calculation formula of the transverse horizontal displacement of the bank slopes of a super-high arch dam caused by the reservoir hydraulic pressure was derived by elastic theory. Based on FEM, the reservoir type of the bifurcated channel was simulated and sensitivity analysis of the influencing factors on the reservoir basin and the dam was conducted. Theoretical analysis and numerical calculations indicate that the main influencing factors of the valley shrinkage are the reservoir water level and the bank slope angle. The distance from the river bifurcation to the dam and the reservoir water level are significant factors affecting the deformation of the reservoir basin and the dam. With the increase of elevation, the transverse deformation of the bank slope below the water surface increases on both sides of the banks and then decreases in the valley direction, while the bank slope above the water surface always deforms in the valley direction.