In this paper, a novel two stage heating system of hydrocarbon fuel was used to simulate the fuel temperature, pressure and fuel residence time in cooling channels of an actively cooling scramjet, wherein the high pressure pyrolysis of n dodecane, a surrogate fuel, was experimentally studied. The fuel cracking experiment was carried out in the temperature range of 830 930 K and at a relatively constant pressure of 40 atm. When the pyrolysis temperature was below 890 K, the main pyrolysis products were macromolecular olefin. With the increase of pyrolysis temperature, the cracking degree of n dodecane and the proportion of small olefin and alkane increased, and the cracking degree reached 53.5% at the pyrolysis temperature of 930 K. The proportion of each gas product is almost constant in the whole temperature range except the lowest temperature of 830 K, while the proportion of each liquid product to the total liquid products shows the same phenomenon. A preliminary detailed chemical kinetics model of 145 substances and 2024 reactions was established with RMG. The reaction mechanism generated by RMG and several published n dodecane cracking reaction mechanisms were used in CHEMKIN simulation, respectively. By comparing the results of the experiment and the kinetic model, the obvious differences were observed and reasons were analyzed. On this basis, the RMG kinetic model was further optimized, and achieved in good accordance with the experimental results. The decomposition pathways of n dodecane were further illustrated through the analysis of reaction flux. The primary decomposition of n dodecane is mainly through the H abstraction reaction with small molecule free radicals, especially CH3 and C2H5. The H abstraction reaction results into six isomers of n dodecyl. The consumption of n dodecyl is mainly through H shift isomerization and beta scissions, which mainly influence the distributions of pyrolysis products.