Abstract: Severe Pilot-Induced-Oscillation(PIO), which is potentially dangerous, paroxysmal and may lead to losing control of flight, is the principal cause of the frequent accidents in the development of FBW aircraft. This compels us to study its flying dynamic mechanism, find out the corresponding design defects of flight control system, figure out the design methods, tools and work procedures to prevent it. The accidents caused by the PIO of FBW aircraft have actually promoted a new round of consideration to the issue of man-vehicle coupling. This includes advancing more precise definitions and more all-sided method of classification, seeking more effective evaluation criterion which considers the nonlinear factors and can identify the tendency of severe PIO.