Abstract: Segment multistage pumps are widely used in industry, and their radical diffusers are always low in matching the impellers. This paper borrows lessons from the area ratio theory and puts forward a new method to determine the parameters of the guide vane of radial diffusers through the mathematic relation. Based on FLUENT software, numerical investigation is carried on the pump with radial diffusers using the standard model and the SIMPLEC algorithm. The result shows the throat area influences the pump performance greatly, and the area ratio Anderson advised is bigger than it should be. The optimal design on the guide vane can increases the efficiency of the pumps, the best model designed in the paper is 2.3 m higher in head and 1.6% higher in efficiency than the ones designed in traditional ways. Therefore the optimal design is effective. The research results can provide a theoretical reference for hydraulic optimization of segment multistage pumps.