[Objective] Microwave-assisted rock breaking technology shows promising application potential in hard rock tunneling. The effectiveness of microwave irradiation on rocks is significantly influenced by their microstructure, particularly the mineral particle size distribution. Existing studies mostly focus on single mineral components or simple binary combinations, whereas systematic investigations into how the heterogeneity of complex mineral particle size distributions in natural rocks affects the microwave rock breaking remain limited. This study aims to quantify the heterogeneity of mineral particle size distribution through experiments and numerical simulations, and to reveal its influence mechanisms on the thermal-physical response, damage evolution, and mechanical property degradation of rocks. [Methods] Seven groups of standard granite specimens (Φ50 mm × 100 mm) with different mineral particle size heterogeneity coefficients were selected and subjected to microwave irradiation tests. Surface temperature variations of the specimens during irradiation were monitored, longitudinal wave velocities before and after irradiation were measured, and the peak strengths were obtained through uniaxial compression tests. Image processing techniques were used to extract the surface mineral distributions of the specimens, and quantitative indicators characterizing the heterogeneity of particle size distributions were defined and calculated. Using COMSOL software, a multi-physics numerical model coupling electromagnetic fields, heat conduction, and solid mechanics was established. The model precisely reconstructed the real mineral distributions with different heterogeneity coefficients, and simulated and analyzed the dynamic evolution process of the temperature fields, stress fields, and plastic damage zones of the specimens under microwave irradiation. [Results] (1) Thermal response: under identical irradiation conditions, the heterogeneity of mineral particle size distribution significantly affected the thermal response of the specimens. With the increase of heterogeneity, the temperature rise of the rock became more pronounced. The specimen with the highest heterogeneity (H=0.78) exhibited a final temperature approximately 44 ℃ higher than that of the most homogeneous specimen (H=0.34).(2) Damage and weakening: with increasing heterogeneity coefficient, the number of microwave-induced surface microcracks increased significantly. The reduction in longitudinal wave velocity intensified, with a maximum difference reaching 30%. The uniaxial compressive strength loss rate increased from 11.2% to 29.6%, with a maximum difference of 18.4%. These results indicated that the more heterogeneous the mineral distribution was, the more severe the internal damage induced by microwaves and the more significant the weakening effect on macroscopic mechanical performance became.(3) Mechanism: stronger heterogeneity led to more intense temperature gradients and thermal stress concentrations at the interface between strong microwave-absorbing minerals (such as potassium feldspar) and weak microwave-absorbing minerals (such as quartz). This was because specimens with higher heterogeneity contained larger potassium feldspar particles, which had stronger microwave absorption capacity, resulting in a rapid local temperature rise. Both the area proportion of the critical tensile stress zones (>15 MPa) and the area proportion of the plastic zones increased monotonically with the increase of the heterogeneity coefficient. The plastic zones first appeared at the contact interface between potassium feldspar and quartz and expanded over time. [Conclusion] The heterogeneity of mineral particle size distribution is a key microstructural factor controlling the effectiveness of microwave-assisted rock breaking. The defined quantitative heterogeneity coefficient can effectively predict the outcomes of microwave irradiation: rocks with higher heterogeneity are more likely to experience uneven heat accumulation and large interfacial thermal stresses under microwave irradiation, thereby leading to more extensive microcrack initiation, more significant wave velocity reduction, and more significant strength loss. This study identifies the potassium feldspar-quartz interface as the preferential site for damage initiation.