Abstract: The conventional design method of oil cooling system was introduced.Based on the synthetic analysis of the performance of heat exchanger,the heat transfer principle between the decelerator and oil,and the heat balance process of the oil system,an improved design method of helicopter decelerator oil cooling system was established to improve the heat balance temperature calculation and heat exchanger design,meanwhile the new heat balance process under other working conditions were considered to improve the reliability of the design.Based on this improved method,a helicopter decelerator oil system was designed as an example,showing the results consistent with theoretical analysis.