Abstract: The super high arch dams built in China, such as Xiluodu and Jinping, show abnormal phenomenon of reduction of valley width during the initial impounding stage. The arch dam is a high-order statically indeterminate structure, which is very sensitive to deformation of dam foundation, especially non-uniform deformation. Based on the deformation mechanism of valley width and the method for calculating the boundary displacement, the possible deformation of valley width of Baihetan arch dam in the process of initial impounding is calculated and predicted, and its effects on the displacement and stress of the dam are analyzed by using the elastoplastic finite element method. The results show that the two methods are greatly different. From the perspective of change of effective stress and weakening of rock mass materials, the maximum reduction of valley width is no more than 40 mm, and it will not greatly reduce the overall stability of the arch dam, but only causes new stress concentration area on the dam body. However, the arch dam has strong overload capability to the boundary displacement, and the impact of the reduction of the valley width on the stress of the dam is very small. The non-uniform deformation of the foundation is the key factor which affects the dam stress.