The microstructure of carbon fiber–reinforced carbon-matrix composites (carbon/carbon composites) has important effects on its ablation performance. However, the traditional macro-ablation methods have underestimated the ablation recession rate and ignored the influence of microstructure. To simulate the ablation of large-sized structures while accounting for the influence of microstructure, it is necessary to modify these methods. In this work, a thermochemical ablation model for carbon/carbon composites is proposed based on the evolution behavior of their microstructure. The ablation recession rate and surface temperature predicted by this model are in good agreement with the experimental results. Through numerical analysis, we found that the ablation recession rate of the material without carbon fibers is much greater than that of the material containing carbon fibers. The ablation recession rate is influenced by the fiber orientation due to the change in thermal conductivity. The anti-ablation efficiency of carbon/carbon composites can be improved by increasing their fiber radius, radiation coefficient, specific heat capacity, interphase density, and thermal conductivity coefficient. The thermochemical ablation model provides a guide for the design of better anti-ablation carbon/carbon composites.