Abstract: In order to investigate the ablation mechanisms of carbon/phenolic composites which were used in thermal protection system of hypersonic vehicles with blunt shapes, the mathematical model for ablation behavior was established firstly, the model took a variety of energy dissipation mechanisms, including the thermal radiation of material surface, the heat absorption of the solid phase by temperature rise, the heat absorption of matrix pyrolysis reaction, the ejection of high temperature pyrolysis gas, "heat block" effect caused by mass ejection, temperature rise and heat absorption by expansion of pyrolysis gas etc., into account, and the solving of mathematical model was realized by finite element method. Then, the ablation behaviors of carbon/phenolic composites under aerodynamic heating environment that cold wall heat flux was 400 kW·m^-2 and enthalpy was 5 MJ·kg^-1 were forecasted. The results show that during heating period, the depth of carbonized layer in carbon/phenolic composite whose thickness is 20 mm increases continually, the surface temperature reaches 1420 K and the rear wall temperature is 346 K at 100 s, the pyrolysis gas pressure arrives 10.3 atm and the depth of carbonized layer is 7.50 mm. The conclusions obtained can provide supports for the design of thermal protection system in hypersonic vehicles which have long-time and large area thermal protection requirements.