This paper explores the maintainability factors of Line Replaceable Unit (LRU) divisions. For civil aircraft, the quality of LRU divisions directly determines the pros and cons of maintainability design. This paper illustrates the meaning of LRU and introduces the concept of aircraft structural decomposition and the LRU determination process. The flow chat was established. The role of maintainability factors in LRU divisions was analyzed. By considering the relationship between LRU and MTTR, the MTTR impementation based on LRU divisions was introduced. The research in this paper has certain guiding significance for subsequent LRU divisions and maintainability design.