<p>Bentonite serves as an engineered barrier material in repositories for high-level radioactive waste geological disposal. The swelling characteristics and shear strength of bentonite are influenced by various factors, including montmorillonite content and pore water chemistry. Based on the fractal model of bentonite surface, a modified effective stress considering the influence of osmotic suction is proposed. The correctness of this model is verified through various tests, including swelling deformation, constant-volume swelling pressure, and shear strength tests of bentonite in different salt solutions. The experimental results demonstrate a consistent linear relationship between maximum swelling deformation in varying concentrations of salt solution and modified effective stress when plotted on double logarithmic coordinates for a given type of bentonite. The swelling force tends to decrease as the concentration of the salt solution increases. Specimens with the same dry density correspond to essentially the same modified effective stress. The modified effective stress represents the total effective stress experienced by bentonite under osmotic suction in salt solution. Moreover, peak shear strength can be expressed using the modified effective stress to satisfy the Mohr–Coulomb criterion within salt solutions.</p>