During the long period operation of ethylene cracking furnace, ash deposited on the surface of furnace tube and coke formed inside the furnace tube in convection section, resulting in higher flue gas temperature of cracking furnace, lower heat transfer efficiency of furnace tube in convection section, and higher heat loss and energy consumption and lower thermal efficiency of cracking furnace. To solve these problems, a combination of mechanical coke cleaning and chemical cleaning was adopted to remove the ash on the surface of furnace tube and the coking inside the furnace tube in the convection section of cracking furnace, so as to improve the thermal conductivity of tube bundle in convection section, improve heat transfer efficiency, and achieve the desired flue gas temperature and thermal efficiency.