Ethylene is an important basic raw material for petrochemical industry, and CO will be generated during ethylene cracking. Excessive CO in cracking gas will lead to the poisoning of C₂ hydrogenation catalyst and the leakage of acetylene, resulting in unqualified ethylene products or even the interlock shutdown of the plant and affecting normal ethylene production. This paper discusses the generation process of CO in steam cracking unit, and systematically analyzes the causes of the excessive CO in cracking gas according to the actual production conditions of the plant. The main reasons for the excessive CO in cracking gas are the over high COT in cracking furnace, the reduced sulfur content of cracking raw material and the incomplete passivation of furnace tube before feeding. Some corresponding countermeasures are proposed according to the reasons for the excessive CO.