Abstract: In order to research the influence of flow deflection produced at 90° corner of suction section on the performance of multistage centrifugal pump，the first-stage pump with guide vane，inlet and outlet segments was selected from the multistage centrifugal pump to build three-dimensional entity model. Based on SST turbulence model and N-S equation, the numerical simulation was completed. The results showed that the flow at the inlet segment of the impeller deflect obviously to one side, the pressure on different channels of the impeller is uneven. According to the simulation results, the structure of suction segment is optimized, which can reduce the back flow area and uniform the flow at inlet segment. After optimization, the efficiency under 1.1Qd, 1.0Qd and 0.6Qd design conditions is improved by 1.4%, 1.1% and 2.6% respectively. The method gives a new way for the optimization design of multistage centrifugal pump.