Abstract: In order to meet the needs for studying PIO（Pilot-Induced Oscillation） and take serious influence of control system nonlinearity（e. g. clearance and friction, etc. ） on PIO into account, a dynamic structure diagram （see Fig. 2） and an analog structure diagram （see Fig. 3） in longitudinal motion of pilot-control-airframe with nonlinearity have been derived, and a new method which further ap-proa ches to the actual case has been provided for studying PIO problem. Moreover computations have been carried out on a DMJ-3A analog computer for three cases as examples, i. e. the moment arm at normal state（short arm）, the moment arm at trouble state（long arm）, and the moment arm still at trouble state （long arm） without pilot's correction, for a fighter flying at low level and high speed. The results are shown in Fig. 4, Fig. 5 and Fig. 6. It is obvious that they coincide with the actual flight and thereby it is proved preliminarily that the structure diagram and the analog structure diagram（i. e. the computation method for PIO） proposed are not only reliable, but also of value in pra-tical analysis and use. The results obtained also demonstrate that the phenomenon of the longitudinal oscillation with big amplitude for a fighter at low level and high speed is the problem of PIO.In order to get a clear understanding of influence of pilot on the pilot-airplane system, different pilot's transfer functions have been adopted to compute the pilot-airplane system on DMJ-3A analog computer. The results obtained prove that the mathematical model of pilot exerts a great influence on pilot-airplane system（see Fig. 7）.