Abstract: The inertial measurement unit (IMU) is the sensor of the attitude control system. The measurement precision of the transfer characteristics of the IMU directly affects its navigation accuracy. A damper is commonly used for the vibration isolation of the IMU, obviously with nonlinear characteristics. In order to analyze the transfer characteristics of the IMU under various working conditions, this paper simplifies the IMU as a Duffing cubic stiffness model with six degrees of freedom. The differential equations of motion under the base excitation is deduced, and solved by the Runge-Kutta method. The transfer characteristics of the system under free and forced vibration conditions are analyzed. It is shown that the frequency and the amplitude of the system under the working excitation condition of multi-degrees of freedom are lower than that under the excitation of single-degree of freedom. In the working environment of the IMU, the actual test should be carried out in the vibration environment of multi-degrees of freedom.