Good altitude performance of lubrication system ensures flying performance of the helicopter. Both engine and reducer need lubricating oil to cool themselves, to keep stable and decrease malfunction in flying tasks. In this article, a multi-factor coupling calculation method is put forward for altitude performance analysis of lubrication system in “Zhi X” main reducer system, taking into consideration of heat transfer and flow resistance characteristics of every component, off-design performance of the fan and dynamic characteristic of heat exchanger. Performance parameters of each component are calculated, such as temperature, pressure and flux. The results are in good agreement with numerical results of FM2. Then an analysis software for altitude performance of lubrication system is developed for “Zhi X” based on the multi-factor coupling calculation method, which is regarded as a scientific and practical tool for altitude performance prediction and integrated design of lubrication system in helicopter.Copyright © 2010 by ASME