With the progress of the science and technology and the rapid development of the aviation industry, the use of a large number of high-tech airborne equipments greatly enhanced the performance of the aircraft. But it also increased the complexity of the system and the degree of coupling among the equipments, further increasing the difficulty of the aircraft fault diagnosis. Because of the limitation of a single fault diagnosis method, it is difficult to meet the diagnostic requirements of complex systems. Therefore, the integrated fault diagnosis technology of different fault diagnosis methods has become the focus of the aviation research. According to the actual demand of aircraft out-field fault diagnosis, the paper puts forward the fault diagnosis method based on model and fault-tree. The principle is that, according to the characteristics of the out-field modular fault diagnosis and the aircraft design knowledge, the physical model based on LRU (simplified aircraft system model based on out-field replaceable unit) is built to describe the system structure, behavior and function of a particular function. Then the fault events are built aiming at the LRU, and according to the connection between LRU, fault analysis of the logical relationship between events and the establishment of fault propagation paths are achieved. At last the fault tree is composed of the paths through the synthesis algorithm. During the fault diagnosis, the procedure is carried out by the cooperation of the physical model and the fault tree. The physical model will be used as the fault observation window, and the difference between the expected behavior of the system under normal circumstances and the actual behavior is used to judge the occurrence state of the fault events. Then the reasoning process is achieved by the fault tree logic, and the results are presented on the physical model, simulating the fault propagation on the LRUs to determine the suspected failure. Based on this method, the aircraft out-field fault diagnosis system based on model and fault-tree is developed using Dorado platform and G2 development system, and the process is verified by the brake system of a certain airplane.