Mineral composition plays a crucial role, the study of its content is important for mechanical properties and fracture initiation mechanisms of shale reservoirs in northern Guizhou. The mechanical properties and mineral composition of shales of the Lower Cambrian Niutitang Formation in northern Guizhou Province were determined by uniaxial compression, Brazilian splitting, acoustic emission signal acquisition, and X-ray diffraction (XRD), and the influence of mineral composition on the mechanical properties of shale reservoirs was analyzed in depth to elucidate the relationship between mineral composition and mechanical properties of shale reservoirs. Results that when the proportion of mineral components changes from toughness to brittleness, the deformation and fracture stage of shale gradually becomes complicated during the compression process. As the shale brittleness index increases and the clay mineral content decreases, the stress-time variation curve becomes curved and sloped, the compressive strength, tensile strength, and modulus of elasticity increase, and the acoustic emission signal becomes more numerous and intense. The fracture extension patterns of the YY-1, YY-2, and YY-3 series under Brazilian splitting and uniaxial compression tests were axial penetration type, lateral slip type, lateral slip + bottom rupture type, and axial penetration type, respectively.