The invention relates to a method and a device for curing a fiber-reinforced resin-based composite material component by utilizing microwaves. The microwaves which are generated by a microwave source with linearly-adjustable power are guided into a resonant cavity, and the microwaves penetrate through and heat the composite material, so that the composite material can be quickly cured and formed. By adopting the advanced octagonal microwave-mode resonant cavity, the uniformity of an electromagnetic field inside the device can be realized; the interference of the reflection wave on the microwave source can be reduced by adopting an automatic impedance matching system, so that the optimal transmission of the microwave power can be realized. A vacuum pipe connector and a temperature sensor are arranged on the inner side wall of the octagonal multi-mode resonant cavity, a glass worktable is arranged inside the resonant cavity, and the rotation of a ball screw is controlled through a stepper motor to control the back-forth movement of the worktable. A choking groove is adopted to prevent the leakage of the microwave. By adopting the method and the device, the problems of the traditional autoclave forming method that the time for manufacturing the fiber-reinforced composite material is long, the energy consumption is high and the heat is non-uniform can be solved, the curing time can be saved, and the quality and performance of the composite material component can be improved.