Abstract: Bentonite, as a barrier material for high-level waste disposal repositories, acts as a buffer material between waste storage tanks and surrounding rocks. The salt solutions in the cracks of the surrounding rock will affect the peak shear strength of the bentonite and interfere with the stability of the nuclear waste repository. Therefore, the determination of the peak shear strength of the bentonite in salt solutions is of great importance. The peak shear strength of the bentonite affected by salt solutions is analyzed from the perspective of electric diffuse double-layer theory. The concept of modified effective stress is proposed to express the peak shear strength of the bentonite incorporating with the osmotic suction of salt solutions. Based on the fractal model for bentonite surface, the expression for the modified effective stress is related to the osmotic suction based on the stress balance of bentonite in salt solutions. The osmotic suction is calculated by the modified Debye-Hückel equation. Considering the Mohr-Coulomb criterion, the peak shear strength (τ_f) of the bentonite is expressed by the modified effective stress (p^e) in a uniform linear relationship as τ_f=c'+p^etanφ'.