Carburizing is the main damage form of ethylene cracking furnace tubes. In this process, the microstructure of the furnace tube would change and the element diffuses and redistributes. After serving for about 41000h, the radiation section of a plum blossom tube of SC-1 tubular cracking furnace from a petrochemical company was tested and analyzed in this article. Results show that the higher the service temperature, the more serious the carburizing of the furnace tube. In the inner-wall carburized zone of the middle temperature section of the furnace tube with an initial C content of 0.1wt%, the maximum C content reaches 1.83wt% and the number of carbides increases obviously as well as its organizational morphology changes from fine granular to coarse block or chain like and its organizational type changes from single M₂₃C₆ to the coexistence of M₂₃C₆, M₇C₃ and MC type carbides. The Cr and C elements in the carburized zone are mainly concentrated in the grain boundary area in the form of carbides. At the same time, the diffusion of alloy elements causes Cr deficiency in the matrix, and the carbide deficiency zone appears in the subsurface of the inner wall.