The swelling property of bentonite-sand mixtures is important to evaluate the long-term performance of deep geological repository of nuclear waste. Through the comparison of swelling test results of different kinds of bentonite-sand mixtures, we can conclude that, for pure bentonite and bentonite-sand mixtures with low sand mixing ratio, montmorillonite void ratio em and vertical stress ?v relation is a unique line in their bilogarithmic coordinates, and we can predict the volumetric strain after water uptake under a given vertical stress and the swelling pressure at constant volume for mixtures with different sand mixing ratios; while, for the mixtures with high sand mixing ratio, the em-?v linear relation is satisfied only before sand skeleton is formed. When the stress is larger than deviation starting stress, the em-?v relation deviates from the line. Moreover, the larger the sand mixing ratio, the smaller is the deviation starting stress. Before sand skeleton comes into being, sand particles are surrounded by montmorillonite, hence the vertical stress is mainly burdened by montmorillonite, and the swelling quantity at full saturation including swelling deformation and swelling pressure is determined by the montmorillonite content per unit volume; after the sand skeleton is formed, the vertical stress is burdened by montmorillonite and sand skeleton. The deviation starting stress when sand skeleton is formed can be determined for mixtures with different sand mixing ratios according to the concept of sand skeleton void ratio, and the range of sand mixing ratio in which the sand skeleton may be formed can also be obtained.