In condition monitoring of train, because of load change, speed fluctuation and other effect factors, it may cause vibration frequency spectrum migration. If using the common vibration signal acquisition and spectral analysis method to this fault, there will be a result of characteristic spectral smearing, which will lead to misdiagnosis or missed diagnosis. To solve this problem, the vibration mechanism of spectrum-varying fault was discussed. This paper studies a pulse-triggered sampling method of integral periodic signal, the equal angle re-sampling method and other appropriate signal processing method. The feasibility and validity of this method and technology has been verified by the application in Liuzhou Railway Bureau .