Abstract: The gear partial fault's typical characteristic is the modulated sideband.The traditional demodulation technology can distinguish the gears' later period fault characteristic,but it is actually helpless to the incipient fault.In order to withdraw the fault information of the incipient data,and carry on the foreknowledge effectively to the fault diagnosis of the large-scale electromechanical device,this paper centered on a ruptured tooth fault which occured in the near past in a grum gear of a high speed rolling mill,adopted a method based on Empirical mode decomposition technology and combined with the Hilbert envelope demodulation analysis,and it obtained the result which withdrew the early crack breakdown information clearly.The results of the analysis on mechanical early vibration signal of the fault showed that the method based on EMD decomposition technology and envelope demodulation analysis can effectively extract the vibration signal characteristics of the mechanical fault,which is beneficial to finding the hidden dangers ahead.