Abstract: Coral sand is a kind of special marine soil. The special material source and formation process lead to the difference of hydrophysical properties between coral sand and terrestrial sand. The coral sand and quartz sand are set as the control group. Through a series of microscopic experiments, the reasons for the special properties of coral sand are quantitatively studied and revealed. It is found that the microscopic factors leading to the particularity of coral sand include two aspects: particle surface properties and particle shape properties. The surface properties of particles include particle electrification and surface hydrophilicity. The particle electrification shows that quartz sand is more likely to aggregate and agglomerate than the coral sand. The hydrophilicity of surface shows that the hydrophilicity of coral sand is worse than that of quartz sand. The shape properties of particles include the overall shape of particles and the surface roughness of particles which show a piecewise rule. When the particle size is less than 0.25 mm, the coral sand has relatively regular shape and smooth surface compared with the quartz sand. When the particle size is 0.25~0.75 mm, the coral sand has relatively regular shape but rough surface compared with the quartz sand. When the particle size ≥0.75 mm, the coral sand is relatively irregular in shape and rough in surface compared with the quartz sand. Finally, the mechanism of the difference permeabilities between the coral sand and the quartz sand is reasonably explained by the above microscopic conclusions, and the piecewise rules of the influences of particle microscopic properties on the permeability of the coral sand are summarized.