To build up the system model in aircraft maintenance simulation, a system modeling method is proposed based on extended reachability matrix together with component design of AGENT. It can not only solve the problem of the expression of logic relationship among the LRUs, but also can provide the system with redundancy. The method can describe the component's work logic in the system and the effect of component state change on the system. The logic of aircraft system can be abstracted and transformed into the effable object of computer with this method. Through the analysis of the reachability matrix, the component related to the behavior of the signal transmission and the behavior of the service can be defined. At last, the feasibility of the method is verified by constructing a logic function model of Automatic Flight System (AFS).