Abstract: The approach based on model was employed to estimate the unbalance magnitude of rotor system in this paper.The original vibration signal of rotor system must be recorded,then,the differences between online detected unbalance responses and original signals were computed.The equivalent loads on each node were calculated through dynamics equation of rotor system with the differences.The magnitude,location and phase of the unbalance were acquired according to the equivalent load.The vibration responses of the need nodes were estimated according to the response of the finite nodes by the modal expansion technique.The denoise approach based on gauss wavelet was proposed due to the existence of a lot of noise.The results show that the proposed approach in this paper is able to estimate the unbalance magnitude accurately.