Abstract: The nonlinear dynamic characteristic of journal gas bearings(self-acting gas bearing and hybrid gas bearing)-flexible rotors coupling system was studied theoretically by employing the orbit method in this paper.The multiple degree freedom model of the rotor system was established,solving the coupling with the Reynold equation(including the time terms) by applying alternating direction implict(ADI) method.Through numerical simulation,the nonlinear gas film forces,trajectory diagram,phase diagram,frequency spectrum,power spectrum,bifurcation diagram,vibration mode diagram and the equilibrium position diagram were obtained.As for the characteristic of the orbit method employed in this study,the methods of obtaining the nonlinear dynamic parameters were proposed.It indicates that the dynamic characteristics of journal gas bearings-rotor system and whirling instability of the gas film could be depicted commendably.