On the basis of thoroughly analyzing the microwave curing process of glass fiber epoxy composites, this paper combines with electromagnetic, heat transfer and cure reaction kinetics theories, to build a three-dimensional coupled model of electromagnetic field, temperature field and cure degree field, and the model includes two internal heat sources. Based on this model, the simulation of microwave curing process of glass fiber epoxy NOL ring is realized, and the simulation results are compared with the traditional thermal curing process. The comparative studies show that the microwave curing method shortens the time for the composite to reach the curing temperature , and reduces the gradient of cure degree distribution in the composite NOL ring and effectively improves the uniformity of cure compared with conventional thermal curing method. The realization of numerical simulations on microwave curing processes of epoxy composites could provide a feasible method for the study of microwave curing technique.