The flight envelope of modern fighter aircraft has a wide range, and the power density of electronic equipments on board is also continuously increasing. For these reasons, the reliability of ECS (environmental control system) for fighter aircraft must be solved to guarantee pilot's safety and comfort, and to ensure the normal operation of electronic equipments on board. This paper focused on the fault diagnosis method research of ECS for fighter aircraft. A fault diagnosis method based on BP neutral network was developed. This paper firstly introduced the work principle and mathematical model of chosen ECS for fighter aircraft. Then on this basis, the FMECA (failure mode effect and criticality analysis) of chosen ECS was demonstrated. FMECA results showed that heat exchanger and turbine were the two equipments failing most frequently. At the end, data from simulation in four typical system fault modes were used to train BP neutral network and implement fault modes identification. Diagnosis results showed that the outputs of trained BP neutral network agreed well with actual system fault modes. The method developed in this paper can be used to diagnose the typical faults of ECS successfully, and is significant for the reliability research of ECS.