Abstract: Bentonite is selected as a buffer and backfill material for deep geological repositories due to its high expansibility, low permeability, and excellent nuclide adsorptive. In order to study the effect of the pore solution of the surrounding rock on the strength of bentonite, GMZ07 bentonite specimens are saturated and consolidated in Na₂SO₄ solution with different concentrations for direct shear tests. The test results show that as the concentration of the salt solution increases, the cohesive intercept and internal friction angle of the bentonite sample increase, and the strength increases significantly. Based on the fractal model for bentonite surface, the modified effective stress that considers the osmotic suction is introduced to explain the test results. The shear strength of GMZ07 bentonite in Na₂SO₄ solution calculated by the modified effective stress is compared with the measured value, and the two are basically in agreement, which verifies the correctness of the modified effective stress considering the osmotic suction.