The cracking gas compressor in the 220 kt/a ethylene plant of PetroChina Dushanzi Petrochemical Company was started in September 2019 after overhaul, the third-stage outlet flow rate was maintained at around 118 t/h, and the third-stage outlet pressure reached a maximum of 1720 kPa, which is close to the set pressure of safety valve of 1746 kPa, resulting in limited load adjustment of cracking gas compressor and weakened ability to resist fluctuations. In response to the abnormal third-stage outlet pressure of cracking gas compressor and the deviations from design conditions, the operating process parameters and mechanical structure of the compressor were analyzed, and some measures such as setting internal control indicators for compressor outlet pressure, replacing inter-stage gas seals and adding high-pressure nitrogen sealing pipelines were taken to ensure the long-term, full-load and optimal operation of the cracking gas compressor.