To study the mechanical properties of rocks under different water content conditions, sandstone was taken as the research object, and uniaxial compression tests were conducted to analyze the influence of water content on the mechanical properties and macroscopic failure characteristics of sandstone. Based on the 3D laser technology and fractal theory, the structural characteristics of macroscopic fracture surfaces in sandstone were quantified, the degree of deterioration of internal structure was determined, and the relationship between macroscopic fracture surface characteristics and mechanical parameters was established. The results show that the uniaxial compressive strength and elastic modulus of sandstone exhibit a non-linear decreasing trend with increasing the water content. The degradation of sandstone structures induced by water can be divided into three stages: intense growth stage, stable growth stage, and growth stage. It is found that the macroscopic failure mode of sandstone shifts from single shear failure to axial splitting failure. The fractal dimension variation characteristics of macroscopic fracture surfaces can be used to quantitatively evaluate the degree of deterioration inside sandstone and reflect its mechanical properties.