Abstract: This paper presents a practical mathematic model of pilot.The PIO （pilot-induced oscillation）problem of synthetical system of "men-mechanical contrtol system （containing the dynamic characteristics of the mechanical stick system and the nonlinear effect）-aircraft" and "men-control augumentation sy tem-aircrraft" is discussed.The influence upon PIO of the parameter variations in the pilot model and the nonlinear effects of the mechanical control system is analysed. Some good results have been obtained.These results are also useful in the study of the PIO problem of "men-Fly by control system-aircraft" synthetic system.