Abstract: The earthquake induced hydrodynamic pressure analysis of a bridge pier in deep water is based on a rigid reflection boundary of bottom, and the absorption action about flexible medium of bottom on hydrodynamic pressure wave is ignored. Earthquake induced hydrodynamic pressure formulary is established in this paper using radiation wave theory, which can consider the flexible reflection boundary of bottom. The influence of the flexible reflection boundary on hydrodynamic pressure of a bridge pier is analyzed. The results indicate that: the hydrodynamic pressure of a bridge pier will be changed in a special load excitation frequency scope when the flexible reflection boundary of bottom is considered, and the hydrodynamic pressure of a bridge pier will be decreased with the reduction of reflection coefficients. The influence of the flexible reflection boundary on earthquake induced hydrodynamic pressure of a bridge pier can be ignored. The present research provides a theoretical basis for the earthquake hazard refined simulation of a long and huge bridge.