It is demonstrated theoretically and numerically that the flexible rotor-centralized squeeze film damper bearings (SFDB) system with multiple degrees of freedom has the nonlinear characteristics similar to those of the Duffing equation of one degree of freedom. The bcne curve and amplitude limit curve of the amplitude-frequency response curve of the system are derived. The relations among tne abuve three curves and their effects on bistable response are analyzed. It is found that the number of the intersection points of the bone curve with the amplitude limit curve determines the nonlinear forms of the system responses

and the amplitude limit curve is tangential to the amplitude-frequency response curve at the intersection point. Thus it is revealed how the Duffing and bifurcation nonlinear responses are formed. It is also proved that the squeeze film force will not change the rigid critical speed of the original system

but make the amplitude-frequency response curve de-1 fleet. This deflection always has the characteristics of hard spring

that is

the amplitude-frequency response curve will deflect in the direction of larger rotating speed. When this deflection reaches a certain degree

the bistable response occurs and it may appear in the both sides of the undamped rigid critical speed of the system.