Potassium acetate was used as a coking inhibitor to reduce coking during light naphtha cracking on a Cr25Ni35 alloy specimen that had already been used for 8 years. The effects of the mass concentration of potassium acetate on the morphology and structure of coke were investigated by high-resolution scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The results show that the oxide scale formed on the inner surface of the cracking tube after 8 years of service is mainly composed of (Fe, Ni, Cr) spinels and the needlelike intermetallic compound of Cr and Fe. Changes of the surface conditions accelerate catalytic coking. The amount of coke decreases with increasing mass concentration of potassium acetate in a 1-h cracking period. The amount of coke Was found to decrease by about 60% when the mass concentration was 400 ppm. The diameters of filamentous coke were about 100, 60, 45, and 35 nm when the mass concentrations of potassium acetate were 0, 100, 200, and 400 ppm, respectively. However, the gasification reaction was found to have little effect on the length of catalytic coke. The gasification reaction catalyzed by potassium acetate removes the noncatalytic coke surrounding the filamentous coke, and filamentous cokes at different concentrations are carbon nanofibers with a solid structure. Coke is mainly composed of amorphous carbon.