Bispectral analysis can reveal information of quadratic phase coupling of a time series and fits well the demands of characteristic extraction while on-line monitoring wear conditions of a milling cutter. In this study, based on bispectrum and bicoherence extraction of vibration acceleration signals at spindle end of a vertical machining center while milling operation, change rules of bispectrum and bicoherence of the signals were analyzed for the progressive wearing process of the flank wear land of the milling cutters. Research results showed: analysis results were consistent with the actual wearing process of the cutters, bispectral analysis can reflect the progressive wearing process of the milling cutters, and the analysis can provide theoretical basis for on-line monitoring the wear process of a milling cutter while machining operation.