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In order to study the cavitating flow at different stages of cavitation in the pump casing of a centrifugal pump, the topology blocking and structured mesh generation strategies was proposed on the domain of the pump. Taking full account of the near wall region mesh quality, steady and unsteady cavitation simulation of the centrifugal pump has been proceed using the SST k-ω turbulence model combining with Rayleigh-Plesset cavitation model. Unsteady flow of the pump chamber and the force endured by the impeller were obtained under three NPSHa. The results show that cavitation increases the amplitude of pressure pulsation in pump casing sharply. Due to the presence of the wear rings, amplitude of pressure pulsation at front side chamber is greater than back side chamber. Broadband pulsation frequency increase with the cavitation intensifies, especially at axis frequency. Intensification cavitation not only affects the flow pattern of the pump chamber, but also increases the radial and axial forces acting on the impeller.