With the aim to meet the demand of a low-specific speed centrifugal pump for the high efficiency and non-overload characteristics at several specific operation points, a series of optimization were done on a model pump TS65-40-160. An impeller was designed based on the non-overload theory, and by analyzing the inner flow distribution in CFD and verify experiments, several improvements were taken to meet the complicated requirements. After several design improvements and experiments, an optimized scheme was obtained, whose head are slightly higher than the given indexes in the shut-off, designed and maximum flow rate points, and the maximum unit efficiency is increased by nearly 14%, and the pump efficiency also meet the national standard requirement, and the shaft power has the non-overload trend, and the power value at 48m3/h is within the motor power. Through many results comparisons from CFD and experiment, a conclusion is drawn that the simulation error is small and it can meet the requirement for model optimization.