<p>Microwave-assisted rock fragmentation has been demonstrated as a promising approach to enhance shield tunneling performance in hard rock strata, primarily due to its ability to cause extensive thermal damage. However, the heterogeneous fracture evolution of granite under mobile microwave irradiation remains unclear. To address this challenge, this study proposes a high-fidelity electromagnetic–thermal–mechanical (EM–T–M) coupling model. In the electromagnetic–thermal (EM–T) module, an implicit function combined with an arbitrary Lagrangian method (ALE) is employed to solve the thermal field, thereby eliminating error accumulation caused by mesh remapping. The thermal–mechanical (T–M) module incorporates a grain-based model (GBM) to simulate heterogeneous thermal stresses induced by variations in electromagnetic and mechanical properties among different minerals. Temperature transfer between the electromagnetic–thermal and thermal–mechanical modules is achieved through interpolation methods. The results demonstrate that microwave power and waveguide velocity significantly influence temperature distribution, with biotite exhibiting substantially higher temperature increases compared to feldspar and quartz. High-temperature zones predominantly concentrate along the waveguide's path. The fracture mechanism is dominated by thermal stress in granite, leading to a specific three-stage evolutionary trend.</p>