<p>The stability of slopes and tunnels is controlled by rock discontinuities, and the rock discontinuities roughness and the sliding direction play a significant role in shear failure. However, three-dimensional roughness evaluation considering shear directions is scare, and the internal shear fracturing processes, micromechanical mechanisms and failure precursor of rock discontinuities are not well understood. Therefore, this study proposes a novel roughness evaluation index to quantitatively analyze the anisotropic characteristics of rock discontinuities. In conjunction with shear tests, a novel 3D-GBM modelling method considering the micromineral constituent and particle size distribution characteristics of granite as well as the geometric shape of discontinuities was realized. The strength, macro and micro-fracture characteristics, visual anisotropic shear evolution process and microfailure mechanism of granite discontinuities at different roughness and shear direction were investigated. Finally, the spatial and temporal evolutions of AE parameter <i>b</i>-value and magnitude <i>M</i> were further analyzed to reveal the shear fracture precursor of granite discontinuities.</p>