It has been calculated the hydrodynamic pressures of gravity dams subjected to vertical earthquake component in the frame of the Scaled Boundary Finite Element Method (SBFEM) considered the water compressibility and the absorption of the reservoir bottom, and it has given the solution of gravity dam-reservoir interaction not only in the frequency domain but also in the time domain. Numerical examples show the proposed method is capable of the high degree of accuracy. It is concluded that the hydrodynamic pressures caused by vertical ground motion are of special important analysis of gravity dams under the prescribed assumptions. Besides, it also has been studied the hydrodynamic effects of water compressibility and the absorption of the bottom.