The　component　of　gear　vibration　signal　is　very　complex,　when　a　localized　tooth　defect　such　as　a　tooth　crack　is　present,　the　engagement　of　the　cracked　tooth　will　induce　an　impulsive　change　with　comparatively　low　energy　to　the　gear　mesh　signal　and　the　background　noise.　This　paper　presents　a　new　comprehensive　demodulation　method　which　combined　with　amplitude　envelop　demodulation　and　phase　demodulation　to　extract　gear　crack　early　fault.　A　mathematical　model　of　gear　vibration　signal　contain　crack　fault　is　put　forward.　Simulation　results　based　on　this　model　show　that　the　new　comprehensive　demodulation　method　is　more　effective　in　finding　fault　and　judging　fault　level　then　conventional　single　amplitude　demodulation　at　present.