Abstract: The sudden unbalance and accelerating processes under the action of fluid inertia force have been investigated for an over-hung flexible rotor with two disks supported on centralized squeeze film damper.The effects of Reynolds number,and system parameters on sudden unbalance and accelerating responses are studied.It is found that the system parameters have large influence on the sudden unbalance transient process and transient time,while the fluid inertia force is considered.The response route of large solution or small solution depends on the starting rotating speed of the sudden unbalance.The system with fluid inertia force will have better accelerating and decelerating dynamic characteristics.