Aiming　at　the　difficulties　in　diagnosis　for　low　speed　and　heavy　duty　components　of　furnace　top　gearbox,　an　indirect　diagnosis　method　for　vibration　signal　is　proposed　in　this　subject,　through　which　the　vibration　features　of　high　speed　rotating　parts　that　near　input　end　of　gearbox　is　effectively　utilized　and　analyzed　for　fault　judgment　of　low　speed　components　and　a　useful　methodology　is　also　given　for　fault　diagnosis　of　both　furnace　top　gearbox　and　low　speed　and　heavy　duty　equipments.　Since　the　identification　for　all　faults　and　accurate　fault　location　cannot　be　realized　by　using　the　existing　diagnosis　methods,　a　method　of　vibration　analysis　for　fault　diagnosis　to　furnace　top　gearbox　is　presented　to　realize　accurate　judgment　and　fault　location.　It　can　be　found　out　that　if　near　the　basic　frequency　and　double　frequency　of　characteristic　frequency　of　high　speed　components　of　upper　gearbox,　there　were　frequency　spacing　of　fault　characteristic　frequency　of　low　speed　components　of　subordinate　transmission　chain　apparently　showing　up,　which　also　happened　in　low　frequency　range　after　demodulation,　then　the　fault　location　can　be　determined　to　the　low　speed　parts　of　subordinate　transmission　chain.