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A coupling method of finite element and discrete element was used to simulate the process of microwave liberating coarse aggregate under different power conditions, and it deals with the problem that the finite element method can not simulate the cracking process and the discrete element method can not consider the nonuniformity of microwave in the concrete specimens. The liberation ratio was proposed as the quantitative evaluation index, and the influence of microwave power on the liberating effect was analyzed based on the change of the liberation ratio. The mechanism of microwave-assisted liberation was discussed. It was found that the temperature distribution and crack distribution in the test specimens changed with irradiation time and microwave power; the optimal irradiation time and the optimal microwave power were obtained by liberation ratio under different irradiation conditions, and the simulation results were verified by preliminary tests; the coarse aggregate was under tension in both circumferential and radial, while the mortar is under compression in circumferential and under tension in radial under microwave irradiation. Cracks are produced by tensile stress at the junction of coarse aggregate and mortar, then the coarse aggregate are liberated from the mortar.