Common failure modes of dual clutch automatic transmission（DCT）were described. Taking “Car can not drive in the gear” failure as an example, the fault tree was built and analyzed qualitatively, and all minimal cut sets were found out. The core issues in construction of knowledge base and realization of reasoning machine of DCT fault diagnosis expert system, such as knowledge acquisition based on fault tree, knowledge representation, database design with relational data model, forward and backward reasoning based on rules,were focused on. It is shown that DCT fault diagnosis expert system based on fault tree design is reasonable, practical.