Abstract: In the oil cooling system of a high performance helicopter,the low mass flow,high rotating speed and small size axial flow fan design is a key task.This special fan's structure was analyzed,and optimization of the relevant parameters was conducted.To obtain good aerodynamic characteristics,the radial aerodynamic balance equations were adopted to build the fan's rotating blades,which were evolved from the simplified three dimensional flow theories,the constraint conditions and the control vortex theory.The improved loss computing method was used to calculate the fan's efficiency,and to optimize design schemes.There is a good agreement between the design results and the experimental data while the design method is applied to an axial flow fan design for a helicopter.