To improve the reliability of helicopter transmission system design， the fatigue life assessment process and methodology of the tail gearbox casing of a certain type of helicopter are conducted. According to the process of assessing the fatigue life of critical components， a four parameter fatigue life prediction method is adopted to study the average S-N curve parameters of materials and components， as well as the safety S-N curve parameters of components based on reduction factors. The calculation method of fatigue life is given through the cumulative damage rule. Based on the full-scale fatigue test results and measured load spectrum of the tail gearbox casing， as well as the numerical simulation analysis of the stress in the tail gearbox casing， the life of the tail gearbox casing is evaluated according to the safety S-N curve and Miner’s rule. The research results indicate that the key parts of the tail gearbox casing meet the design requirements for infinite lifespan. Among them， the fatigue dynamic strain value and safety fatigue limit of the Part 2 of the tail gearbox casing are relatively large.