In order to reduce the combustion experiments and improve the scramjet structure design, the supersonic flow and combustion process of hydrogen and air in the scramjet are simulated with the large-scale simulation software Fluent. Firstly the cooling mixed flow of air and fuel in the scramjet is simulated, then supersonic combustion process is simulated with one-step or multi-step reduced chemical reaction mechanism respectively. The simulation results show that when using one-step global reaction mechanism, the fuel can be ignited easily, and the combustion efficiency is also relatively high, but the temperature obtained is higher. Simulation with the multi-step reduced reaction mechanism is closer to the actual combustion experiments, but the fuel may be ignited harder, the flame may easy to destroy during the simulation, and the time of simulation may relatively longer.