In deep tunnelling, mining and subsurface energy recovery, the reliable estimate of rough fracture (or fault) strength and the potential for reactivation are of vital importance for the assessment of dynamic geo-hazard, such as fault slip rock-burst and induced earthquakes. In this study, true triaxial loading tests were conducted on pre-fractured granite with different orientations to the maximum principal stress, and the fracture slip process was studied with the aid of acoustic emission and deformation monitoring. The reactivation strength was also compared with the theoretical predictions based on the analytical model. Results show that the critical fracture angle for the rock matrix failure and original macro-fracture reactivation is ~ 51°, above which fault slip occurs along the original macro-fracture at a gradually smaller differential stress. The microscopic analysis indicates that new faulting develops traversing the original fracture which becomes more compacted and closed with insignificant damage when the fracture angle is below ~ 44°. The differential stress required for rough fracture reactivation is well predicted by the single plane of weakness theory. In addition, variation of acoustic emission signals (especially AE energy) and deformation along σ₃ direction with loading time are very consistent, which can be used to analyse the preparatory and evolutionary process of the fracture reactivation. The stress rate decreases whilst deformation rate increases both from a steady state value to a very large value as the fracture is gradually reactivated, accompanied by the b value decreasing from 1.4–2 to 0.4–0.6, which can be used as precursors for the dynamic fault slip. The findings in the present study will provide new insights into the mechanics of rough fracture reactivation in deep tunnelling, mining and underground resource extraction.