Abstract: Based on nuclear magnetic resonance technology, crack development laws of granite were analyzed under triaxial compression condition, the stress-strain curves, T₂ spectrum curves and porosity under axial pressure of 20 MPa were obtained. The results showed that T₂ spectrum distribution reflected the process of crack development visually and factually, and the porosity increased exponentially with axial stress. When the granite was in the low axial compression stage, rock damage was mainly caused by the increase of fracture number; under the high axial pressure, rock damage caused by both fracture number and crack connection. In the triaxial compression process, the axial stress threshold of granite was present, the new fractures were inhibited and the existing cracks have been extended to the penetration when it exceeded that limit, the axial stress threshold of granite in this experiment was 50% to 70% of peak strength. The variation characteristics of granite damage degree with rock porosity and axial stress accord with exponential function relation under confining pressure of 20 MPa.