This paper introduces a diagnostic maintenance system ITMDMS that combines expert system technology with the conventional testing technology. The system includes two kinds of knowledge: one is heuristic shallow knowledge, the other is deep knowledge. They are represented by first-order predicate logic in hierarchy structure. The system takes three steps to accomplish the inference process: (1)shallow inference; (2) deep inference; (3) shallow inference. The knowledge maintenance methods perfect the knowledge of the system in applifìcation. The frame of the system can be used in diagnostic maintenance of any digital system.