Abstract: The active cooling technology of endothermic hydrocarbon fuels is a key way to solve the thermal protection of high-speed aircraft engines, but the condensation coking problem during engine shutdown is a bottleneck that affects the reusability of aircraft. In this study, a self-designed apparatus was used to separately analyze the condensation coking during the fuel cooling process, and the coking characteristics under different temperature conditions were obtained. The condensation coking mechanism of fuel during cooling process was proposed based on the changes in physical properties of coking precursors obtained by the group contribution method. When the temperature drops to 300 °C, not only the gas yield and conversion increase to 71.42% and 89.75% respectively, but the coke mass on the inner surface of the tube also significantly increases from 0.39 to 1.92 mg. Meanwhile, as the temperature further decreases, the morphology of coke gradually transforms into amorphous carbon with a higher degree of graphitization. During the cooling process, due to the liquefaction of coking precursors, their physical properties such as viscosity, density, and saturated vapor pressure undergo sudden changes at 300 °C, leading to enhanced intermolecular physical interactions and promoting the physical aggregation of coking precursor molecules, which are deposited on the inner wall of the tube. This work provides a theoretical basis for the subsequent study of condensation coking mechanisms and inhibition methods.