<p>Unbalanced force produced by the unbalanced mass will affect vibrations of rotor systems, which probably results in the components failures of rotating machinery. To study the effects of unbalanced mass on the vibration characteristics of rotor systems, a flexible rotor system model considering the unbalanced mass is proposed. The time-varying bearing force is considered. The developed model is verified by the experimental and theoretical frequency spectrums. The displacements and axis orbits of flexible and rigid rotor systems are compared. The results show that the unbalanced mass will affect the vibration characteristics of rotor system. This model can be more suitable and effective to calculate vibration characteristics of rotor system with the flexible deformation and unbalanced mass. This paper provides a new reference and research method for predicting the vibrations of flexible rotor system considering the unbalanced mass.</p>