Abstract: Space mechanical cooler is the key part of the infrared detector module.While the performance parameters meet the requirements，reliability becomes the key factor limiting applications in real missions of the cooler.Based on deferent requiement of on-orbit life，the derating strategy of the cooler is the effective way to increase reliability in orbit.This paper summarizes the cooler machine failure mode and analyzes its failure mechanism，identifies the derating basis，and determines the derating factor value.