The optimization of the start-up process of ethylene plant is crucial to reducing flare emissions. Before the cracking furnace is fed, the nitrogen cycle of cracked gas compressor is used to start the pre-cooling of separation cold zone, and the temperature in cold box and demethanization system is reduced to -110～120℃. A new ethane cracking furnace is installed to increase the steam generation during the start-up, and the nitrogen pre-cooling is carried out at higher pressure, so that the outlet pressure of the cracked gas compressor can reach the allowable value of the feed to separation system without feeding materials. While the pre-cooling time is shortened, the time required for pressurizing the separation system is reduced, greatly reducing the flare emissions during the start-up.