After seven months of operation since the overhaul of cracking gas compressor in the 200 kt/a ethylene plant at PetroChina Liaoyang Petrochemical Company, the loss of lube oil increased significantly. It was judged that the leakage failure of the four-stage mechanical seal of the compressor high pressure cylinder was the cause. To solve this problem, the engineering and technical staff analyzed the problem from the perspective of equipment manufacturing and maintenance quality, process operation stability and production fluctuations, identified the fundamental reasons for the mechanical seal failure, and proposed some improvement measures to ensure the stable, safe and long-term operation of the compressor.