Abstract: In order to set controller parameters correctly, a graphical stability analysis approach for active queue management (AQM) is proposed. The model of TCP/AQM is converted into a second-order system with time delay. The stability of the closed-loop AQM system is described in terms of characteristic quasi-polynomial. New necessary and sufficient stability criterion is deduced based on the inverse Nyquist curve and the negative frequency characteristic line. The relations between stabilizing boundary of proportional gain in PID controller and network parameters are investigated. Different stabilizing regions are compared to show the less conservatism of our approach, and simulation experiments implemented by both Matlab and Network Simulator validate our analysis. The specialty of the proposed approach lies in the lower complexity of the calculation procedure and intuition in the complex plane.