The strength of rocks is typically affected by elevated temperatures and confining pressures. In this investigation, tri-axial tests under various orientation angles, temperatures, and confining pressures are performed on the anisotropic rock, slate. Then, the discrete element method is utilized to duplicate the slate's mechanical and failure properties in the same lab setting. The simulation results of the DEM models exhibit U-shaped curves in peak strengths, aligning well with empirical findings. Orientation angles significantly influenced the failure characteristics and mechanical performance of the slate. Additionally, strength anisotropy in the slate showed minimal response to slight temperature changes. It was demonstrated that the detrimental effects of high temperatures on bond weakening were alleviated by the application of elevated confining pressures. Additionally, by properly adjusting the microparameters of the parallel contact and smooth-joint model, the proposed models accurately represented the anisotropic mechanical characteristics of the slate at different foliation orientations. Finally, the proposed thermal weakening function for parallel bond strength effectively estimated changes in the strength and failure patterns of slate under different thermal states, particularly between temperatures of 25 °C and 100 °C. This study significantly advances our comprehension of slates’ thermal–mechanical behavior in most underground projects and geothermal development.