At present, ethylene plant is generally started up in the reverse sequence. Propylene refrigeration compressor and ethylene refrigeration compressor are started first, and then cracking gas compressor is started and nitrogen is introduced. Nitrogen is used as the medium and ethylene and propylene are used as the cold agent to precool the cryogenic systems such as cold box and demethanizer, and ethylene and propylene are introduced to achieve the total reflux operation of ethylene rectification column and propylene rectification column. After the precooling of cryogenic systems, natural gas is introduced into the cracking gas compressor to pre-start the separation system in advance, so as to get through the whole process from cracking gas compressor to cold box, demethanizer, deethanizer, high-pressure depropanizer and C₂ hydrogenation reactor.