Affected by the changes in raw material structure, the 150 kt/a ethylene plant at PetroChina Jilin Petrochemical Company had problems such as N factor increase of cracking gas compressor, coking in discharge line and cavity, increased discharge pressure at the second stage and over high speed and vibration of cracking gas compressor. By injecting dispersed scale inhibitor, the operation could be maintained for a period of time. However, with the worsening of coking, the unit was forced to be shut down for maintenance. By studying the polymerization mechanism, this paper puts forward that the optimization of cracking raw materials and the control of compressor inter-stage temperature are the key to prevent polymerization.