Abstract: In order to find out the influence mechanism of sand particle size on the mechanical properties of similar materials and improve the accuracy of similar physical simulation experiments, 99 mechanical specimens under 11 particle size conditions of 0.1-3 mm are tested for uniaxial compression and permeability coefficient.The law and mechanism of the mechanical properties of the specimens are explained from two aspects:gypsum hydration reaction mechanism and internal and external structural characteristics.The study finds that the influence of sand particle size on the compressive strength of similar materials ranged from -16.51% to 49.79%, but the overall particle size has a negative correlation with the material strength; SEM electron microscopy finds that in smaller sand grains the CaSO₄·2H₂O crystals formed under the diameter condition are mostly intertwined needle-like and short-thick rod-like crystals, and the material strength is also large.When the particle size is large, it is mostly in the form of a sheet or a plate, and the degree of interlacing is poor, and the mechanical strength is weak; characteristic observation shows that the smaller the sand particle size is, the smaller the length and width of the interface joint is; the larger the specific surface area of the sand is, the more uniform the contact stress is, and thus the less stress concentration to destroy the material.The permeability coefficient of similar materials increases with the increase of sand particle size; when the particle size is between 1.5 mm and 2.0 mm, the uniaxial compressive strength of the test piece is closest to the target value.