Excessive lags of aircraft could cause Pilot Induced Oscillations(PIO).The frequency domain method and time domain method were used combinedly for studying the relationship between excessive lags and PIO quantitatively.A simplified closed-loop aircraft-pilot system model was built up,and the aircraft-pilot system stability was analyzed.In frequency domain,the frequency characteristic of effective aircraft was investigated by using Bode plot and Nichols plot, and quantitative estimation was made to the relation of the effective time delay and PIO by using bandwidth criterion and phase rate criterion.In time domain,the influence of excessive lags was quantitatively studied by using pilot induced oscillations time history and pilot rating.The results can provide a quantitative reference for controlling excessive lags of aircraft.