Abstract: The influencing matrix of absorption boundary conditions of reservoir bottom in fluid-structure system is derived, and the dynamic coupling reactions of the dam is calculated with different reflection ratios. The resultshows thatthe dynamic responses ofthe damare related closelytothe characteristics ofthe reservoir bottom. The greater the reflection ratio is, the larger the dynamic responses of the dam and the hydrodynamics of the water in the reservoir are. Therefore, ignoring the absorption of the reservoir bottom will result in unacceptable errors.