Abstract: To separate multimode Lamb waves overlapped in time and frequency domains,a multimode Lamb wave signal separation method by using fractional differential is proposed.The Tsallis function is used as the model of the amplitude spectrum.The amplitude spectra of Lamb wave signals are divided into fractional order differential.The characteristic parameters of each mode are extracted by using the polynomials between the maximum amplitude and the derivative order as well as those between the peak frequency and the derivative order.The corresponding phase spectrum is derived from the dispersion characteristics.The signal of each mode is constructed through the amplitude spectrum and phase spectrum.The test results of simulated S₁ and A₁ modes in a 1 mm-thick steel plate show that the relative errors of amplitude and bandwidth of both modes are less than 6%,and that the relative errors of central frequency of both modes are less than 0.25%.Therefore,this method could be used to separate the time-frequency overlapped multimode Lamb wave signals when the amplitude spectra of the mixed signal are not completely overlapped.It is helpful for the analysis and recognition of multimode Lamb wave signal.