Abstract: When deep-buried water diversion tunnels pass through altered rock formations, variations in water content may cause deterioration in the surrounding rock strength, posing risks to structural safety. To investigate the shear behavior and morphological evolution of altered granite under different moisture conditions, large-scale direct shear tests were performed on altered granite samples from a deep-buried tunnel in Guangdong Province under five water content levels (from dry to saturated). Combined with particle size distribution and quantitative analysis of shear surface morphology, the regulatory mechanism of water content on shear strength was revealed. Results indicate that shear strength reaches its maximum at 10% water content, where cohesion first increases and then decreases (peak at 64.5 kPa), while the internal friction angle decreases continuously (by 70.7%). At high water content, a muddy zone forms on the shear surface, the fine particle content increases by 37%, and the residual strength drops by over 35%. The study identifies a dual degradation mechanism of pore water pressure and weakened mineral cementation, and proposes targeted support and moisture control strategies for altered rock sections of deep-buried tunnels. The findings provide theoretical guidance for stability evaluation and disaster prevention in complex hydrogeological environments.