Abstract: The styrene exhaust gas compressor was taken down because of the high vibration of its gearbox.After many overhauls,the fault was still not eliminated.By using the online state monitoring system-system 1,this paper makes an analysis on the vibration spectrum from four aspects including unbalanced force,even unbalance,dynamic unbalance and assembly size.According to the vibration characteristic reflected by the vibration axis locus diagram,the dynamic unbalance is finally diagnosed as the cause for the high vibration and the interlocking of the gearbox.The maintenance plan is formulated again at site.After online and offline dynamic imbalance elimination,the fault is resolved and styrene exhaust gas compressor runs well.