Abstract: The compressible effect of water is often neglected in the simulation of hydraulic machinery. However,based on experimental and numerical study, it is found that the compressibility of water could influence the magnitude of the pressure pulsation at some frequency in the pump. Therefore, in order to investigate the influence of water compressibility, compressible model is established by using Tait equation. The internal flow of centrifugal pump under different conditions is calculated by this model. The calculated results are compared with the incompressible results, and it is indicated that the compressibility of water has little effect on the performance parameters. But it affects the amplitude of pressure fluctuations at some discrete frequency, especially at the outlet of impeller and volute tongue where significant jet-wake and rotor/stator interaction appears respectively. Meanwhile, water compressibility makes greater influence on the flow pulsation under off-design condition. Therefore,it is necessary to consider the compressibility of working medium in the numerical simulation of unsteady flow in centrifugal pumps, especially in area with strong unsteady flow and at off-design condition.