Abstract: The transition of polyacrylonitrile (PAN) molecules to graphite-like structures is at the heart of the carbonization of PAN-based carbon fibers. Studies of the laser-induced carbonization of pre-oxidized PAN fibers can greatly help in revealing the carbonization mechanism. The differences between the laser-induced and heat-induced carbonization of fibers at 2500 K were investigated by reactive molecular dynamics simulation. The energy evolution, chemical reaction rates, the fluctuation of atom position and forces on atoms were analyzed. Results indicate that the simulation using a ReaxFF reactive force field can provide detailed information for the reactions, which is helpful for insight into the formation of graphitic structures in the carbonization. The thermal shock stress caused by rapid laser heating in the carbonization results in an amplification of the fluctuation of atom positions, which increases the number of active atoms and the atomistic activity. As a result, the probability of collision and chemical reaction between atoms is increased, which promotes the formation of graphite structures. A more graphitic structure is achieved by the laser irradiation heating of PAN-based carbon fibers compared with traditional heating.