To explore the influence of the compressibility of reservoir water on the dynamic response, we carried out time-history analysis on the seismic dynamics of Wugachong arch dam using 3-D finite element model. Compared with the additional mass model, the model in the present study can better reflect the influence of reservoir water on the dynamic response. In the additional mass model, since the compressibility of reservoir water was not considered, the natural frequency of arch dam was lower and the hydrodynamic pressure was exaggerated. Changes of transverse and vertical displacements were little but the longitudinal displacement was increased, and the change of  stress in arch direction was little while the vertical stress was increased. 