To accurately simulate the hydrodynamic pressure on a gravity dam face during an earthquake, a computing method considering both the flexibility of dam body and reservoir bottom is proposed based on the seamless coupling theory of finite element method and scaled boundary finite element method. By using the scaled boundary finite element method, semi-infinite impounded water can be modeled with high efficiency and accuracy with only discretization of the interface of the reservoir and the dam. In addition, water compressibility and the effect of absorption from reservoir bottom can be conveniently considered. Finite element method is employed to discretize the dam body and the dynamic response of the dam-reservoir system can be solved by the hydrodynamic pressure on the dam face. Numerical examples show that the hydrodynamic pressure calculated by the proposed method agrees well with that in the literature. The hydrodynamic pressure on a gravity dam face can be reduced by dam flexibility and it decreases with a decreased reflection coefficient of the reservoir bottom.