Abstract: To analyze the closedloop stability of the phaseplane jet control system, a method is proposed to obtain the numerical describing function, by which the effects of control variables on the frequency domain properties can be studied. An analysis method is given to decide the stability and the stability margin of the closedloop system. An instance is given to illustrate how to apply the given methods to predict the stability properties of a designed phaseplance thrust control system effectively. The advantage of this technique in redesigning the phaseplane controller and achieving better robust properties are also illustrated by another example.