Abstract: Based on theoretical analyses and experiments of flexible rotor squeeze film damper system with centralizing springs,this paper presents the investigation of the main resonant bistable operation occurred near the main resonant speed of the system,and deals with the regularity of occurence of the bistable operation and the influence of system parameters on the behaviors of the bistable operation.The calculated results and the experimental data are in a good agreement.It is found that the main resonant bistable operation is characterized by the sudden change of rotor vibration amplitude and by the transposition disc mass center.The reasons of the difference between theoretical analyses and experiments are discussed.