Abstract: In this paper the author identifies that the rotor system will be nonlinear provided the support stiffness varies at ? with the amplitude unchanged and no external load is added to the support (it is difficult to add rotating load to it). Its amplitude-frequency curve must be obtained in accordance with the nonlinearvibration theorem. Although the maximum amplitude of the rotor system with nonlinear support stiffness may be decreased, it never follows the curve of solid line in Fig.l as the addition of two curves with different linear support stiffness. In order to demonstrate this argument, a brief analysis of the vibrationcharacteristics of a nonlinear rotor system is completed and its amplitude-frequencycurve is plotted in Fig.4.