Abstract: Microwave, as a highly efficient, clean and controllable rock-breaking technique, exhibits considerable application potential in the permeability enhancement and stimulation of deep shale reservoirs. Bedded shale was selected as the research object in this study, focusing on its physico-mechanical response characteristics under different microwave irradiation durations. Taking the non-uniform temperature field induced by electromagnetic-thermal effects as the bridge, an electromagnetic-thermal-mechanical coupled numerical model for bedded shale was established by integrating the finite element method (FEM) and the discrete element method (DEM), which systematically revealed the thermal field response behavior of bedded shale under microwave irradiation, as well as the damage and fracture effects induced by microwave irradiation. The results indicate that: (1) Microwave irradiation induces a non-uniform temperature field within the shale, and the coefficient of variation (CV) of the shale surface temperature exhibits a three-stage evolution pattern characterized by “rapid increase-slow increase-stabilization” with increasing irradiation duration; (2) Under the simulated condition of 1 kW microwave irradiation for 180 s, microwave-induced thermal microcracks effectively deteriorate the mechanical properties of bedded shale, resulting in a maximum reduction of up to 55% in uniaxial compressive strength and a 10%-20% decrease in elastic modulus; (3) Microwave irradiation promotes the transition of the macroscopic failure mode of shale from brittleness to ductility, with the most pronounced effect observed at a bedding angle of 60°—under 1 kW microwave irradiation for 180 s, the brittleness index decreases by approximately 25%; (4) With increasing microwave irradiation duration, the connectivity of the fracture network and the overall fragmentation degree are significantly enhanced, while the crack propagation mode gradually transforms from a “bedding-dominated mode” to a “thermal damage-bedding jointly-dominated mode.” This study reveals the deterioration effect of microwave irradiation on the mechanical properties of bedded shale and the associated damage and fracture mechanisms via numerical simulation. The findings are expected to provide scientific basis for microwave-assisted unconventional energy reservoir exploitation technologies.