Abstract: To investigate the influent of impeller trim on hydraulic performance of the radial diffuser in a centrifugal, the three-dimensional turbulent flow in a cut impeller of a single stage in multi-stage centrifugal pump MD-280-42×4 was numerically simulated using a SIMPLE algorithm based on Navier-Stokes equation, and the standard k-ε turbulence model. The external characteristic and the fluid flow of the radial diffuser in a centrifugal pump were analyzed. Research results show that with the size of impeller trimmed under the design conditions (Q_d = 0.078 m³/s) increasing, The hydraulic efficiency and shaft power of the radial diffuser in a centrifugal decline. With the size of impeller trimming under the same small flow raising, the fluid flow of both gap between the impeller and the radial diffuser and the anti-guide becomes more and more uniform, and the hydraulic efficiency is enhanced in the same case; With the size of impeller trimming under the same mass flow increasing, the fluid flow of the guide vane and the anti-guide is badly interrupted, and the energy dissipation raises in the both positions. The hydraulic efficiency reduced in the same case.