Abstract: The bistable behaviors of resonance type occurred near main resonance speed of the system is investigated,based on experiments of a flexible rotor-squeeze film damper system. The influences of unbalance speed,radial clearance ratio, centralizing spring stiffness and acceleration process on the behaviors are considered. It is shown that the squeeze film damper is effective only within a certain unbalance range. If the unbalance is large enough, the bistable operation may occur. The larger the unbalance, the centralizing spring stiffness or the radial clearance ratio is, the more the possibility of occurance of the bistable operation.