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.