The controllability of a magnetorheological(MR) fluid squeeze film damper under a sinusoidal magnetic field was experimentally studied on a flexible rotor. It is shown that the frequency of the excitation magnetic field has a great effect on the controllability of the MR fluid damper. As the magnetic field frequency increases, the controllability of the MR fluid damper significantly reduces. There is a maximum frequency of the magnetic field for a given magnetic field strength or a minimum strength of the magnetic field for a given magnetic field frequency to make the dynamic behavior of the MR damper be controllable. When the magnetic field frequency is over the maximum one or the magnetic field strength is less than the minimum one, the controllability of the MR fluid damper almost completely disappears and the dynamic behavior of the MR fluid damper with the sinusoidal magnetic field is the same as that without the magnetic field.