Abstract: With a nonlinear non-steady oil-film force model newly developed by Zhang Wen et al for short bearings, the complicated motion of rotor-bearing system with pedestal loosened on one side was analyzed using numerical simulation and other methods. The motion in periodical, quasi-periodical and chaotic forms in the system, which take place while the speed ratio changes, was discussed. The results show that a triple-frequency motion is found even though in the higher speed range if a pedestal is loosened in the system. Moreover, peculiar moving path of loosened journal center and phase path of slightly shaking pedestal is found. A theoretic base is thus provided to diagnose effectively the fault of pedestal looseness in a rotor-bearing system.