The diffuser grid was mounted at the impeller inlet to improve the anti-cavitation properties of high-speed magnetic drive pump. The fluid field of two high-speed magnetic pump, with the diffuser grid and not, was numerical simulated by Fluent program. The distribution of pressure was taken out, and the trace of the particle was plotted. The main characteristics of the interior fluid field of the two pumps were obtained by comparing with two groups of data. The results show that the pump with the diffuser grid of changeable position in axial direction can improve the anti-cavitation performance. The application examples from practical tests favorably validated the results of simulation, which provide theoretical basis for improving the design.