This paper will use case-based reasoning and rough set methods to conduct commercial aircraft fault diagnosis research. MongoDB is used to store the aircraft fault case, and the constructed Chinese aviation wordbook is used to replace synonyms to reduce the difference in the Chinese expression of the case description. The rough set is used to represent the case, attribute weights by rough set and hamming distance are used to case retrieval, and verify the feasibility and effectiveness of the method by the instance case. Finally, case library learning improves the reliability of case-based reasoning system.