Abstract: After the arch dam is built and impounded, the change of water environment will change the mechanical properties of rock mass in the dam site area, and then cause valley shrinkage, affecting the safety of the dam. Aiming at the abnormal deformation of the foundation of Jinping Ⅰ arch dam at the initial stage of impoundment, based on the law of rock mass deterioration in water and the basic theory of unsaturated seepage stress coupling, we established a model of rock mass deterioration in water, used nonlinear finite element numerical analysis method to study the valley deformation law under the rock mass deterioration in the reservoir area at the initial stage of impoundment, and further analyzed the impact of valley contraction on the dam structure. The study shows that the deterioration of rock mass on the reservoir bank has a direct impact on valley deformation. The higher the deterioration degree of rock mass, the more obvious the valley width shrinkage, and the valley deformation in the upstream fluctuation zone of the dam body is the most prominent. With the increase of water level, the deterioration range of reservoir bank rock mass expands, and the maximum displacement along the river, the maximum principal tensile stress and the maximum principal compressive stress of the dam body slightly decrease. Therefore, valley deformation caused by rock mass deterioration after impoundment of Jinping Arch Dam will not affect the overall safety of the dam.