Design and optimization of high performance external fans are important to improve the efficiency of large and medium high efficiency motors. We propose an optimal design method of centrifugal fans with backward blades by analyzing the characteristics of centrifugal fans. Taking a YXKK710 4 high efficiency motor as the object, the performance and working point of the centrifugal fan are analyzed, and the effects of the centrifugal fan on the efficiency, noise and temperature rise of the motor are tested. The results show that in comparison with the original design, the optimized centrifugal fan is closer to the working point, and the performance data such as efficiency and noise are better. The rationality and reliability of the proposed method are verified. This research can provide guidance for the improvement of motor performance and efficiency, and for the development of series high efficiency motor products as well.