Nowadays, the depletion of shallow resources drives deeper mining operations. Microwave pre-treatment has shown promise for efficient drilling in deep hard rock. While previous studies have confirmed the feasibility of microwave-assisted crushing of hard rocks and analyzed their structural and mechanical property changes at various scales, the microscopic mechanisms behind the evolution of the macro-mechanical parameters of hard rock remain unclear. This study addresses this knowledge gap. At the microscopic scale, the mineral characteristics and the micromechanical properties of minerals (including interfaces) at different sites before and after microwave irradiation were tested in typical hard granites. At the macroscopic scale, the real-time monitoring of mass and surface heating-rupturing characteristics of granite during microwave irradiation was achieved. Meanwhile, acoustic wave and uniaxial compression tests were conducted to explore the evolution of the macroscopic physical and mechanical parameters of granite before and after microwave irradiation. Variability in the mineral structure and mechanical properties accounts for differences in the uniaxial compression strength of granites. To realize the macro-micro linkage, the micro-mechanical parameters of minerals in different granite sections before and after microwave treatment were upscaled. The upscaling results, obtained using the Mori-Tanaka method, closely matched those from uniaxial compression tests, and the upscaling of mineral micro-mechanical parameters in interior samples was found to accurately predict the weakening of macro-mechanical properties of granite. This study provides insights into how microwave irradiation affects the mechanical properties of granite at a microscopic level, offering a quick and efficient method for assessing microwave weakening in deep hard rock and establishing a theoretical foundation for microwave-assisted mechanical drilling in industrial applications.