Abstract: Rock serves as a typical discontinuous medium material, and its mineral composition characteristics result in a large number of particle interfaces, pores, and primary fracture networks inside. This multi-scale heterogeneous structure makes it prone to physical and chemical deterioration under weathering, significantly changing its strength characteristics and deformation behavior, thus threatening the stability of engineering rock masses. This study took the black shale from a lead−zinc mine as the research object. Through a series of laboratory experiments on black shale specimens with different degrees of weathering, the deterioration law of the mechanical properties of black shale during the weathering process was obtained. The results show that the uniaxial tensile strength, uniaxial compressive strength, elastic modulus, and cohesion all decrease sharply with the increase of weathering time, generally showing a negative exponential relationship, while Poisson’s ratio shows a power-law relationship with the weathering time. Cumulative damage to rock from weathering reduces the specimen’s coefficient of friction, leading to the initiation and propagation of microcracks within the rock at lower stress levels. The research results provide a theoretical basis and scientific evidence for the stability assessment of rock masses in slope engineering.