Abstract: The power curve of low specific speed centrifugal pump sharply rises with the increase of flow rate, the ratio of maximum power and power of design-point is much larger than the corresponding value of general centrifugal pumps. This feature could easily cause supporting electrical overload in large flow area. In order to achieve non-overload performance and ensure the reliability of pump operation, the IS50-32-160 non-overload centrifugal pump was optimally designed by changing geometrical parameters of the impeller and clogging part of flow passages. Software Fluent was used to research the relationship between the ratio of effective area between the inlet and the outlet of the flow passages between two blades which was called FII/FI, and the performance of the low specific speed centrifugal pump. When the quarter of flow passages were clogged and FII/FI was 1.17, the efficiency curve was smooth and the extreme value appeared in large flow area of the shaft power curve，then the non-overload performance came true. The method can effectively improve the hydraulic performance of non-overload pump on the premise of the non-overload performance.