In 2019, the capacity of a megaton ethylene plant was expanded in the overhaul window period, including the installation of an ethane cracking furnace, a viscosity reducing tower system and a toluene extraction system as well as the transformation of related supporting equipment and facilities, so that the capacity reached 1.15 million tons per year. This paper analyzes and summarizes the operation of the ethylene plant after two overhauls in 2015 and 2019 respectively, in order to find out the operation bottleneck of quench system and formulate targeted optimization plans.