The seal characteristic is a significant factor affecting the motion of the rotor system. The correct description for the gas excitation force is much important to study the rotor-seal system. Linear seal model is quite hard to accurately describe the motion characteristics of the rotor-seal system, so this paper proposes a new model of the nonlinear seal force based on the two-control-volume model and the motion equations of the rotor to investigate the rotor system. To study the influence of the seal force on rotor-seal system, this paper analyzes the rotor orbit, Poincare map and bifurcation diagram through adopting the new model. This paper gives a reference to further research the motion characteristics of the rotor-seal system.