<p>The presence of water in rocks significantly affects the stability of underground rock masses. This study investigates the pore structure, mechanical properties, energy evolution, microstructural characteristics, and mineralogical changes in red sandstone at varying moisture contents (ranging from 0 to 3.712%) through Nuclear Magnetic Resonance (NMR), uniaxial compression tests, Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) experiments. The aim is to examine the influence of water on the macro- and microstructural properties of sandstone, as well as the associated damage mechanisms. The results indicate that as moisture content increases, the porosity and fractal dimension of the sandstone increase significantly, leading to greater complexity and irregularity in the pore structure. The evolution of the pore structure exhibits a critical point, particularly when the moisture content exceeds 2.784%, beyond which pore connectivity improves and the proportion of large pores increases. The compressive strength and elastic modulus of the sandstone decrease significantly with increasing moisture content, while the peak strain increases. The failure process becomes more gradual and shifts toward a more ductile behavior. Water reduces the energy storage capacity of the rock, as reflected by a decrease in elastic energy at the peak stress point and a significant increase in dissipated energy. At the microstructural level, the softening, swelling, and dissolution of clay minerals promote pore expansion and the development of microcracks, which are identified as the fundamental causes of the deterioration in the mechanical properties of the sandstone. A functional relationship between porosity, fractal dimension, and mechanical parameters was established, quantifying the macro- and microstructural damage of the rock. This relationship accurately captures the trend of changes in its mechanical properties.</p>