With the continuous development of hypersonic aircraft towards high Mach and wide speed domains， the propulsion system faces low dynamic pressure working conditions， leading to a huge challenge to the flow mixing in the combustion chamber. The influence mechanism of hydrocarbon fuel preheating on supersonic shear mixing characteristics in the scramjet combustor was studied， and the influence of temperature and hydrocarbon fuel pyrolysis on mixing characteristics was analyzed. It is found that the preheating of non-cracking hydrocarbon fuel will enhance the viscous dissipation of the jet and reduce the mixing efficiency. When the fuel temperature increases from 750 K to 900 K， the mixing efficiency of the combustion chamber will decrease by nearly 5%， and the total pressure loss will increase by about 20%. However， the thermal cracking reaction of hydrocarbon fuel has a dual effect on the shear mixing. The expansion performance of the cracked hydrocarbon fuel is improved， and the mixing efficiency near the nozzle is increased by about 18%； However， due to the decrease in turbulent kinetic energy of the fluid in the shear layer， the mixing efficiency decreases by about 6% at a position far from the nozzle.