Abstract: Carbon/silicon carbon (C/SiC) composite is used as a new type of thermal protection material for near space hypersonic vehicles. The influence of preparation process on ablation resistance has been studied by performance experiments at home and abroad. The traditional theoretical models of ablation resistance are based on liquid oxide film. Recently, the heating experimental results in tubular furnace and microscopic morphology of sample characterized by electron microscopy show that the oxidation of C/SiC composite can form a porous solid oxide film under atmospheric pressure when the temperature is below 1 696 K. The pore characteristics of the outer oxide layer were evaluated by mercury intrusion method. Based on the gas diffusion behavior in the pores and reaction dynamics of oxidation, a new passive oxidation model of C/SiC composite was established. The predicted values of the model are in good agreement with the experimental results, which indicates that the new passive oxidation model has good prediction ability for the thickness of oxidation film and mass loss of material.