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In order to investigate the flow loss in guide vanes, detached eddy simulation (DES) was used to simulate the transient flow field of a multistage centrifugal pump. The obtained numerical results were compared with experimental results, and the outcome showed that mean errors of head and efficiency were less than 4%. Simulation results analyzation showed that there were three main causes of the loss, which were flow separation in return guide vane passage, circumflux in transferable channel and collision in PS side. In order to decrease the loss, return guide vanes were designed through impeller blade design methods and transferable channels were optimized. The results showed that flow separation and collision reduced when applied impeller blade design method and circumflux reduced through TZ type transferable channel, which improved pump’s efficiency by 2.4% and 1.7%, respectively. The analysis of flow fields in the coupled vanes showed that circumflux, collision and separation flow were both reduced significantly, which improved pump’s efficiency by 2.9%.