Three-dimensional numerical simulations were performed to study the internal flow of a mixed-flow pump impeller under design conditions, by using commercial CFD software - NUMECA, specifically developed for the flow analysis of turbo-machinery. The agreement between the calculated results and the experimental data proved that the adopted numerical methods and models were reasonable and accurate. By analyzing the calculated distribution of the static pressure and the velocity, the internal flow characteristics of the impeller were revealed that there were some flow problems caused by unsuitable local structural design, such as the vortexes and flow separations. Then in order to solve these problems, the blade profile optimization method was confirmed and made through a multi-parameter, multi-objective optimization process. The optimization results showed that the vortexes and the flow separations were apparently weakened, and the efficiency was improved by 4.2 percentage points. The obvious optimization effect indicated the optimization process was accurate and effective.