A two-solid-phase cell ablation model involving an oxidative gas phase, a carbon fiber and a carbon matrix is proposed and the Boltzmann equation of non-equilibrium statistical physics was numerically solved to simulate the spatial evolution of each phase at the mesoscopic scale during the diffusion-surface reaction process. The predicted results of the model are in a good agreement with those of the analytical solution. A more complex three-solid-phase cell model that considered a fiber/matrix interphase was numerically simulated. The simulation results show that the roughness of the composites after oxidative ablation mainly depends on the morphology of the interphase between the fiber and the matrix. The hypothesis of a linear distribution of the concentrations of gas species is reasonable only away from the interphase, but not near the interphase. A maximum ablation depth exists due to diffusion limitation near the fiber/matrix interphase, which depends on parameters such as the Sherwood number, the fiber radius and the reaction rate ratio of the fiber to the interphase with the oxidative gas. This interphase tracking algorithm has good stability during numerical calculations.