Phenolic resin is a matrix material widely used in ablative thermal protection systems (TPS) and its pyrolysis behavior is crucial for the analysis and prediction of TPS thermal response. However, it still remains a huge challenge to elucidate the involved reaction mechanisms due to the process complexity and unpredictability. Herein, a combination of TG-FTIR and ReaxFF MD simulation method was adapted to explore phenolic resin pyrolysis mechanism. Experiment results indicate that the pyrolysis process of phenolic resin can be divided into three stages, and the activation energy of each stage increases gradually. Heating rate affects the initial pyrolysis temperature of phenolic resin, but has no obvious effect on the types of pyrolysis products. Additionally, an accurate and reasonable decomposition kinetic model is established based on 30 kinds of kinetic function models, involving diffusion, random nucleation and nuclei growth, phase interface reaction and chemical reaction. The cook-off simulations results suggest that the major pyrolysis products of phenolic resin at high temperature include H₂, CO, C₂H₂, and H₂O, which formation mechanisms are also discussed according to the simulation trajectories. The reaction model and the activation energy of phenolic resin pyrolysis based on simulations are in good agreement with the experimental results, contributing to the deep understanding of phenolic resin pyrolysis behaviors at atom scale. This study may provide theoretical data basis for the establishment of thermal response calculation model of resin-based composite thermal protection materials.