Abstract: The mechanical properties of rock are of great significance to the selection of cutting teeth and the efficiency of tunnel excavation in rock mass engineering, and the mechanical properties and microstructure of rock change considerably under different moisture content. To investigate the variation in mechanical properties of rock under different moisture content and enhance the effectiveness of engineering excavation, three types of shaly sand with different moisture content in the mining area are selected as the research objects, and moisture content and water absorption tests, splitting tensile strength experiments, and uniaxial compression tests are carried out to study the changes in rock mechanical parameters. On this basis, the mineral composition of the rock is explored by XRD experiment, the microstructure of rock is examined by SEM experiment, and the microscopic reasons for the different moisture content of the rock are analyzed. The SEM images are quantitatively analyzed by Image J software to study the influence of rock microstructure on mechanical properties and to analyze the influence mechanism of water on the internal structure of rock. The experimental results show that with the increase of moisture content, the hydration effect of water on rock becomes more obvious, and the higher the moisture content, the weaker the tensile strength of rock. The difference in moisture content makes the stress-strain curve of rock change differently. With the increase of moisture content, the effective stress transfer area of rock decreases, the elastic modulus and Poisson’s ratio of rock decrease, the peak stress gradually declines, and the tangent of the curve flattens after the peak stress. The mineral composition of the rock affects its moisture content: the lower the quartz content, the higher the proportion of illite, and the higher the moisture content. Moisture alters the microstructure of rock. With increasing moisture content, the number of pores in rock increases, the pore diameter enlarges, and the structure becomes looser. Through the above experiments, the influence of moisture content on the physical and mechanical properties of rock and its underlying reasons are analyzed, which helps reduce water-related disaster accidents caused by groundwater at the source. The research in this paper holds certain engineering significance for selecting cutting teeth and improving tunneling efficiency.