The new gas phase cracking furnace in the ethylene plant of a petrochemical company adopts HQC’s large-scale gas phase cracking furnace technology, double furnaces are designed, and all the heat is supplied by bottom burner. After the gas phase cracking furnace was completed and put into use, there was an abnormal situation of uneven thermal field distribution in the radiation section, which was manifested by unstable flame shape, large COT deviation and easy fluctuation, and low ethylene yield. The thermal field distribution of the gas phase cracking furnace was observed and the causes were analyzed, the burner was modified twice, the thermal field distribution of the cracking furnace was optimized, and the ethylene yield of the gas phase cracking furnace was improved.