Abstract: In order to solve the problems in the design optimization of automobile cooling water pump impeller, such as long design cycle and unclear optimization method, a design and optimization method of semi-open impeller under rated operating condition was proposed based on the theory of centrifugal pump design and computational fluid dynamics（CFD）. Firstly, the structural parameters of the impeller were determined, and the water pump was preliminarily modeled by the 3 D modeling software CATIA V5. The design model was simulated and analyzed by using the CFD software STAR CCM+, and the absolute pressure cloud diagram, velocity vector diagram and turbulent kinetic energy distribution diagram of the pump section were obtained. Finally, the main structural parameters of the impeller, such as inlet and outlet widths, inlet and outlet diameters, inlet and outlet angles, were optimized by MATLAB genetic algorithm toolbox with the aim of minimizing the total energy loss of the pump. The results of the study show that MATLAB genetic algorithm toolbox plays a significant role in the process of pump optimization through the simulation comparison of the cooling water pump before and after optimization under multiple operating conditions. In addition, MATLAB genetic algorithm toolbox has ob-vious advantages in finding the optimization combination of impeller structural parameters, and the results are reasonable and reliable. The pump performance after impeller optimization is excellent, and the pump efficiency has been increased by 2.3% under rated operating conditions.