In order to research the relationship between the size of impeller cutting of super-low specific speed centrifugal pump and the changing pump performance, centrifugal pump IS 50—32—250 was chosen as the research object. By using commercial software Fluent to conduct numerical simulation of three-dimensional flow field, the distributions of pressure and velocity in the impellers and pump bodies under different impeller diameters were calculated. By comparison of calculated results of different diameters, the influence of the size of impeller cutting on pump performance was obtained: when the size of impeller cutting was 0.03, the efficiency raised suddenly; when the size of impeller cutting was 0.09, the efficiency decreased slightly; when the size of impeller cutting was 0.15, the efficiency sudden dropped dramatically. The size of impeller cutting could not be bigger than 0.15 in the practical application, otherwise, the performance of pump dropped dramatically.