Rocket nozzles go through extreme high-temperature heating at the working state. Ablative materials are mostly used to insulate the metallic housing and maintain the internal contour. The ablation process is more complicated when it comes to the effects of pyrolysis reaction of the resin. Recent researches mostly studied the ablation-pyrolysis mechanism using the heat and mass transfer coefficients, ignoring the real diffusion of pyrolysis gas and different reaction model of fiber and resin at the meso scale. In this work, a thermochemical ablation model of porous carbon fiber based on the lattice Boltzmann method is established, in which the flow, heat transfer, heterogeneous chemical reactions and the evolution of solid composites are comprehensively considered. Moreover, the effects of gas flow temperature, velocity, and concentration on the ablation process are discussed. Taking into consideration of the effects of phenolic resin pyrolysis around carbon fibers on the ablation process, an ablation-pyrolysis coupling model is further developed. The results show that, increasing incoming flow velocity will promote the worm holing phenomenon. While the matrix pyrolysis extends 30.0% ablation time due to huge heat absorbed by pyrolysis reaction. This work potentially enhanced mesoscopic mechanism understanding of ablation process and promote lightweight design for fiber/matrix composites.