There is a large temperature difference in the tow cross-section during the graphitization of carbon fibers via laser, which leads to a serious fuzz phenomenon of carbon fiber tows. When the laser exceeds a certain power, the phenomenon of tow breaking may even occur, seriously affecting the continuous graphitization of the carbon fiber via laser. The use of microwaves and lasers to synergistically graphitize carbon fibers can reduce the temperature difference and thermal stress of the tow during graphitization. In view of the particularity of microwave heating, a heating cavity that can achieve continuous and efficient temperature rise is designed; then, a multiphysics model of electromagnetic heating--laser heating--thermal radiation--flow heat transfer--solid mechanics is established for simulation. The simulation results show that the introduction of microwaves can improve the distribution of the temperature field of the tow during laser graphitization and effectively reduce the thermal stress during tow heating.