Coking tests were carried out on the inner surface of after-service HP40 cracking tube at 850℃ with naphtha as feed，and the anti-coking properties of anti-coking SiO2/S coating and sulfur phosphorous coking inhibitor were investigated.The effect of SiO2/S coating and sulfur phosphorous coking inhibitor on coke formation was studied.Scanning electron microscopy (SEM)，X-ray energy dispersion spectrometry (EDS)and X-ray diffraction (XRD)were used to characterize the morphology and structure of the inner surface of after-service HP40 cracking tube and the morphology of coke on the surface.The structure of coke was analyzed by TGA，elemental analysis and Raman spectroscopy.The results showed that the changes of the inner surface conditions of after-service HP40 cracking tube led to accelerated catalytic coking. When the mass fraction of sulfur phosphorous inhibitor was 200 μg·g-1，coke deposition was reduced by 69%.SiO2/S coating could inhibit catalytic coking，and coke was mainly deposited in the spalled SiO2/S coating region.Coke formation was reduced by 65% for SiO2/S coating only.When the combination of sulfur phosphorous coking inhibitor and SiO2/S coating was applied，the interference of HS radicals，originating from sulfur phosphorous inhibitor decomposition，with the heterogeneous noncatalytic mechanism in the cracking via radical termination and hydrogen abstraction reactions resulted in an optimum mass concentration of sulfur phosphorous coking inhibitor.The combination of sulfur phosphorous coking inhibitor at the mass concentration of 50 μg·g-1 plus SiO2/S coating would result in a decrease in coke formation up to 97%.Coke deposited on the inner surface of after-service HP40 cracking tube had a highly condensed structure with a high boiling point and a low graphitization degree.The use of SiO2/S coating and sulfur phosphorous inhibitor led to a decrease in the graphitization degree of the coke to a certain extent.