This article elaborates on the fluctuation of spent caustic oxidation unit in ethylene plant, and analyzes the operating parameters of caustic wash tower and the sampling of caustic liquor and spent caustic in each section of the caustic wash tower. Based on the sampling results, the reasons for the fluctuation of spent caustic oxidation unit were analyzed, and it was determined that the fluctuation mainly came from the black solid particles in the spent caustic of the 220 kt/a ethylene plant. Based on this, some corresponding preventive measures were proposed.