In the ethylene plant of PetroChina Dushanzi Petrochemical Company, 5C Ⅱ centrifugal compressor is the key equipment of air separation unit, and the fluctuations in compressor unit directly affect the stability of air separation unit. This paper introduces the maintenance of Level 3 vibration anomaly of 5C Ⅱ compressor unit to reversely infer the reasons for the abnormal vibration of compressor unit, comprehensively discusses the operation, process analysis and maintenance treatment of the compressor unit, and puts forward the operation bottlenecks of 5C Ⅱ compressor in use, such as the accuracy of inlet air filter core, the internal inspection of intermediate heat exchanger, and the processes of inspection, maintenance and re-commissioning.