Abstract: In order to explore the cavitation mechanism and the development process of cavitation cloud of centrifugal pumps, the unsteady cavitating gas-liquid two-phase flow in IS125-100-200 centrifugal pump was simulated numerically using OpenFOAM7.0 open source software under the working condition range of 0.3 Q_d-1.4 Q_d(where, Q_d is the flow of centrifugal pump under the design conditions). Specifically, the flow velocity, pressure and cavitation cloud distribution in the centrifugal pump were obtained by means of post-processing. On this basis, the relation between the Net Positive Suction Head available(NPSHa) and the occurrence of cavitation cloud in the centrifugal pump under the design condition was analyzed. Meanwhile, the influence of cavitation under the design condition on the internal flow field of the centrifugal pump was also studied by monitoring the pressure coefficient at the key points in the centrifugal pump. In addition, the relationship between the flow of centrifugal pump and the Net Positive Suction Head required(NPSHr) was predicted based on the simulation results of the flow field, and then comparison was made with the measured data. The results show that it is feasible to numerically simulate the cavitating flow of the centrifugal pump under the design condition with OpenFOAM open source software. The cavitation clouds in the blade channels are distributed asymmetrically at different NPSHa under the design condition of 1.0 Q_d. Definitely, the cavitation clouds develop downstream in the impeller channel continuously with the decrease of NPSHa and appear on the blade pressure surface. Regardless of whether cavitation occurs in the centrifugal pump or not, the pressure coefficient shows the periodic pulsation, and the pulsation number of the impeller in a rotation cycle is equal to the number of blades of the impeller. Generally, the research results could provide an effective calculation method of open source software for the performance prediction of centrifugal pumps.