The stability of surrounding rock during deep tunnel construction is strongly influenced by groundwater. To clarify this effect, granite specimens with varying degrees of water saturation were subjected to uniaxial compression tests, while acoustic emission (AE) signals were recorded in real time. The results indicate that both uniaxial compressive strength and elastic modulus decrease progressively with increasing saturation. Failure modes also vary with water content: dry specimens are characterized by brittle tensile failure, low-saturation specimens by brittle shear failure, and highly saturated specimens by distinct plastic deformation. Increasing water saturation substantially suppresses AE activity, leading to lower AE counts. The proportion of high average frequency (AF) and low rise time/amplitude ratio (RA) signals decreases initially and then increases, while the frequency range of AE signals in the late loading stage narrows, with low-frequency signals becoming dominant. These AE characteristics are attributed to water-induced microstructural degradation of granite and to the scale-dependent transition of fracture mechanisms. The findings provide valuable insights for stability assessment and safety monitoring of surrounding rock in deeply buried tunnels of plateau railways.