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In the large flow conditions, the double suction centrifugal pump is susceptible to cavitation damage and has bad operation reliability. Aiming at the problem, a compound impeller with long and short blades was used to improve the cavitation characteristics. By using the shear stress transport (SST) k—ωturbulence model and Rayleigh—Plesset cavitation model, numerical simulation of the double suction centrifugal pump with a conventional impeller and a compound impeller were carried out respectively. The simulation results in the form of characteristic curves were compared with the experimental data, and the internal flow field was analyzed. The result showed that in the large flow conditions, critical net positive suction of the pump with a compound impeller was reduced. Also, the compound impeller can reduce the vapour distribution, change the pressure distribution and improve the dropping flow and backflow. Moreover, the better effect would be achieved under large flow condition.