This paper introduces and analyzes the decrease of interstage outlet temperature, the abnormal operation of caustic washing and spent caustic treatment unit, the fluctuation of process parameters during washing oil injection, and other abnormal phenomenon caused by the insufficient separation efficiency and liquid entrainment of demister in the interstage tanks of cracking gas compressor during the full-load operation of the 1.1 Mt/a ethylene plant of a company. Through design modification, the demister in the interstage tanks was completely replaced during the major overhaul in 2024, and the operation of the cracking gas compressor system before and after the replacement of demister was compared. The results show that the upgraded demister can meet the full-load operation requirements of the ethylene plant, and all the abnormal situations existing before the modification have been resolved.