Determination of the hydrodynamic pressure acting on dams during earthquake has been of high interest in design steps of these structures. An efficient method for the analysis of forced vibration of the dam and reservoir in different modes of the dams motion and deformation has been presented which, could be easily extended to study fluid structure interaction. The SBFEM (Scaled Boundary Finite Element Method) is utilized to model the far field sub domain while the FEM (Finite Element Method) is used to model the near field sub domain of the reservoir. Although a simple geometry of boundary conditions has been chosen, all the required necessities to extend the method to a practical case are implemented.