Ethylene cracking gas main valve assembly is one of the key equipment in ethylene cracking furnace. This paper, based on the engineering example of a newly constructed ethylene plant, discusses the role and importance of the stable operation of cracking gas main valve for the entire process of cracking furnace, elaborates on the optimization of purge steam line for the main valve, and discusses the layout of outlet pipeline of the main valve as well as two design schemes for the support configuration of the main valve.