The degree of salinity in the hydraulic fracturing rejection fluid of the Shaximiao reservoir in the central Sichuan Basin is high, and the underlying mechanism causing this salinity is not clearly understood. We evaluated the rock structure of tight sandstone, including rock composition, pore structure, ion diffusion, and adsorption behavior, to determine how the rock structure influences the mechanism of the sandstone’s interaction with the fracturing fluid. X-ray diffraction revealed that the rock mineral fraction has a significant clay mineral concentration. The results of linear swelling experiments revealed that the water sensitivity of tight sandstone reservoirs exhibits moderately robust characteristics. The time required for salt ion diffusion stabilization is much longer than that required for self-imbibition stabilization, and the diffusion of salt ions is almost log-linear with time after imbibition stabilization. The diffusion rates of salt ions were determined for different single minerals and particle sizes, with clay minerals and particle sizes controlling the diffusion rates. The samples were treated with different concentrations of KCl and acrylamide polymer solutions, and both the pore size distribution and pore throat properties were characterized by low-temperature nitrogen adsorption. Although the adsorption isotherms of both KCl and polymers are consistent with the Langmuir model, their adsorption mechanisms acting on tight sandstone and the effect of adsorption on the pore throat structure are inconsistent. The adsorption of potassium chloride enhances the rock’s pore throat diameter and permeability by increasing the uniformity of the pore throat by electrostatic adsorption with clay particles. However, polymer adsorption reduces the pore diameter of the rock and forms a thin film that obstructs the pore throat, complicating the pore throat and weakening its permeability. This research has led to a greater comprehension of the ion diffusion characteristics of the tight sandstone in the Shaximiao reservoir and the adsorption mechanism on the pore structure of the rock.