Thermal cracking is easy to cause the coking during the regenerative cooling process in the high Mach number scramjet. It is necessary to study the thermal cracking behavior in the cooling channel. A one-step thermal cracking global reaction was established based on the experimental results of reaction products. Using the global reaction kinetics a 3D numerical simulation of thermal cracking coupled with two-phase flow and heat transfer was carried out in a cooling rectangular channel. Three cases of different heat fluxes were compared between the simulation and experiments. The results show that wall temperature distribution of the simulation only using two-phase model are in good agreement with the experiment data before the thermal cracking reaction. There is an obvious difference between the simulation and experiment when fuel temperature increases above 450℃. The higher the fuel temperature is, the bigger the difference is. The wall temperature of the simulation after introducing thermal cracking model decreases and agrees well with the experimental results. When heat flux increases, the cracking reaction rate and conversion rate of kerosene grow. The temperature distribution of kerosene in a curved channel is asymmetric.