Abstract:Abstract: A nonlinear dynamic model of rotor - seal - bearing system with ultra-large-scale dimension for an actual steam turbine is proposed, in which the non-linear seal-excited force, the elastic support force of tilting-pad bearing, the damping force, imbalance force and gravity of rotor are considered. The unbalance response of the rotor is numerically calculated by the Newmark method. The typical characteristic of self-excited vibration and threshold speed of instability induced by the seal-force were revealed. The effects of system parameters on dynamic response and instability were also investigated. The results show that appropriately increases of rotor damping, seal’s radius clearance and axial velocity of seal fluid can improve the stability of the system by increasing the threshold speed of instability. The results can be used for improving the stability of rotor-seal-bearing system in the designing and operating of large steam turbine.