This study introduces a coupled electromagnetic–thermal–mechanical model to reveal the mechanisms of microcracking and mineral melting of polymineralic rocks under microwave radiation. Experimental tests validate the rationality of the proposed model. Embedding microscopic mineral sections into the granite model for simulation shows that uneven temperature gradients create distinct molten, porous, and nonmolten zones on the fracture surface. Moreover, the varying thermal expansion coefficients and Young’s moduli among the minerals induce significant thermal stress at the mineral boundaries. Quartz and biotite with higher thermal expansion coefficients are subjected to compression, whereas plagioclase with smaller coefficients experiences tensile stress. In the molten zone, quartz undergoes transgranular cracking due to the α–β phase transition. The local high temperatures also induce melting phase transitions in biotite and feldspar. This numerical study provides new insights into the distribution of thermal stress and mineral phase changes in rocks under microwave irradiation.