In order to improve the thermal protection efficiency of carbon/phenolic (C/Ph) composites used as the heatshield of hypersonic vehicles, it is essential to find out the components of its thermal protection mechanism. In this study, a pyrolysis layer model considering pressure is established to simulate the percentages of heat dissipated by different thermal protection mechanisms. Numerical results reveal that the thermal protection mechanism of C/Ph composites mainly comprises of surface radiation, the heat absorbed by the heat capacity and thermal blockage effect (TBE) of pyrolysis gas, but the TBE weakens as heating time increases. Furthermore, the evolution laws of different thermal protection mechanisms of C/Ph composites with resin content are given. Based on the components of thermal protection mechanisms, both reducing the thermal conductivity of the resin matrix composites and increasing the material emissivity are effective approaches to enhancing the thermal protection efficiency of C/Ph composites exposed to severe aerodynamic heat.