FEM models of two different scales, including the reservoir basin and near-dam area, were established based on study of the method for determining the model boundaries. By extracting the boundary condition of the near-dam area model from deformation results of the reservoir basin model, a reservoir basin effect analysis method is proposed to study the effect of reservoir basin deformation on the deformation behavior of high concrete dams. Calculated results of a typical project show that reservoir basin deformation has a significant effect on the deformation of a high concrete dam with a large reservoir capacity. In the study of the operational performance and back analysis of mechanical parameters and deformation monitoring indices of high concrete dams with measured data, especially for the project with a wide area of water surface in front of it and a large reservoir capacity, the effect of reservoir basin deformation should be taken into consideration so as to fully understand the operational performance of dams, and effectively monitor the dam operation in safe conditions.