Fault　vibration　signals　of　gear　box　are　faint　shock　and　are　hardly　extracted,　which　bring　a　big　difficulty　for　diagnosis.　This　paper　mainly　introduces　the　basic　principle　of　the　resonance　demodulation　technique　and　its　application　study　in　the　gear　box＇s　fault　diagnosis.　The　developed　resonance　demodulation　technique　can　effectively　extract　the　fault　features　from　complex　vibration　signals　through　the　resonance　demodulation　technique　according　to　the　vibration　features　of　the　gear　box,　and　improve　the　equipments＇　fault　diagnosis　precision.