Double-volute casings were introduced to reduce the radial force that is inherent in single-volute designs. A general three-dimensional computational fluid dynamics（CFD）simulation of a double-volute centrifugal pump was presented to predict hydraulic performance and impeller radial force. By using the shear stress transport (SST) k—ω turbulent model, the simulation results in the form of characteristic curves were compared with the experimental data. The result presented that the head and efficiency of the original double-volute pump drop were 21.8% and 41.3% respectively at design point due to the failed dividing rib (splitter) in the volute casing. Thus, with the aim to analyze the effects of rib structure on pumps, three optimized rib schemes were designed to reduce the impeller radial force and keep the hydraulic performance. Three factors were considered in the rib design: start point, end point and curve equation. According to the steady numerical simulation result in each optimized scheme by CFD, No.2 scheme reduced about half of the radial force while keeping the hydraulic performance. A method in designing the rib structure for double-volute centrifugal pumps was provided.