Special attention is given to the Dam and reservoir interaction in all analysis because the safety of Dam during an earthquake is a vital issue for downstream parts. In this paper the interaction of dam-reservoir is evaluated in the case of earthquake. Therefore the hydrodynamic pressure in the upstream face of the dam should be calculated. For this regard, the governing differential equation on hydrodynamic pressure waves should be evaluated by the consideration of boundary conditions. Therefore we will concentrate on upstream boundary condition modeling which is fulfilled by use of two different methods: finite elements method with Sharan boundary condition and scaled boundary finite element method. In this research, fluid is considered to be homogeneous, non-viscose and compressible and is divided into two different regions: near and far. For the modeling of Dam body in the near field, finite element method and for the modeling of far field, scaled boundary finite element method is used. A concrete gravity Dam is used for this regard and it should be mentioned that two dimensional analysis is mostly used for the evaluation and modeling of such Dams.To be innovative, scaled boundary finite element method is used for the modeling of reservoir and to solve the governing equations on fluid and structure, simultaneous method of equation solving is used.In accordance to the fact that scaled boundary finite element method could model an infinite media, therefore complete attenuation is modeled and accurate results are presented. This method could shorten the calculations and increases the accuracy.