Abstract: A nonlinear oscillator model,which is usually used to study the dynamic characteristics of structures in flow,has been applied to the nonlinear dynamic characteristics of rotor/seal system.Emphasis is put on the effects of the nonlinear unsteady flow in the small clearance on the rotor dynamic characteristics.The mechanism of anti-swirl arrangements for suppressing rotor/seal instability is considered.The comparison between the theoretical and experimental results of a rotor/seal system is in a reasonable agreement and shows that this method provides a theoretical basis for application of anti-swirl technique to engineering practices.