Abstract: Based on the vibration signal and the theory of the second-order time synchroextracting transform, the energy ratio of the harmonic frequency and the energy ratio of the main harmonic frequency are proposed to meet the needs of condition monitoring of milling cutter abrasion. Three T45 milling cutters with different wear states were used for corresponding milling experiments. The results show that the proposed indicators can effectively evaluate the severity of milling cutter wear and can be used to monitor the wear status of milling cutters.