Flight simulator is a complex man-in-the-loop (MIL) simulation system. With several decades of development, it has already become important tools of aircraft design and development, and necessary means of pilot training. And simulation credibility and reliability of the flight simulator have been considerably improved in comparison with the past. However, the system of flight simulator has become increasingly complex and difficult to be described clearly. This paper presents the concepts of conceptual layer and achieving layer, analyzes the composition and principle of the commercial aircraft flight simulator for pilot training from the two layers, describes the system architecture in detail. According to the system architecture, three aspects are very important to develop the flight simulator, the first is the fidelity of the simulation models, the second is the performance of cueing devices, and the last are the computing capacity of the host computers and the time delay over the communication networks.