Abstract: A Porous Squeeze Film Damper(PSFD)had been developed by the authors based on the normal Squeeze Film Damper(SFD).The outer race of the PSFD is made of permeable metal material.Its permeability allows some of the oil in the clearance of the damper to seep in or out through the outer porous race for remarkable improvement of the squeeze film damping properties.In this paper,the steady state response and the transient response resulted from sudden unbalance in the case of blade loss are investigated for simple rigid rotor and a flexible Jeffcott rotor supported on concentric PSFD or SFD.The nonsynchronous response of the simple rigid rotor is analyzed.The investigations show that the nonlinear vibration characteristics with the cavitated SFD systems such as bistable jump phenomenon,nonsynchronous response and "lock up" at rotor pin pin critical speeds under much high unbalance level are greatly improved or avoided with the PSFD systems.PSFD systems could be operated efficiently at higher shaft speeds and under greater unbalance loads especially under greater sudden loading of blade loss.As a new damper based on SFD,PSFD retains all the advantages of SFD and provides more effective vibration control for the rotor systems.