The mixture of bentonite and sand is an ideal buffer material for deep landfill disposal of nuclear waste. The swelling deformation of bentonite-sand mixture is an important parameter in evaluating the barrier system of disposal repository engineering. In this investigation, the swelling deformation of sodium bentonite mixed with sand was examined through compaction tests, loaded expansion ratio tests, and expansive force tests. Furthermore, the influencing factors on expansive deformation were analyzed in terms of microstructure via scanning electron microscopy. Results show that with the increase of sand dosage, the maximum dry density of bentonite-sand mixture also increases; also the compaction curve gradually evolves from being relatively flat and open to vertical and narrow, which means that the variation of dry density gradient of the mixture is affected markedly by moisture content. The expansion process of the mixture, in terms of the time curve, can be divided into three stages: the rapidly expanding stage, the decelerated expanding stage, and slowly expanding stage. Moreover, the relation between loaded expansive rate δ and time can be fitted by hyperbola; the relation between maximum expanding force and sand dosage follows exponential distribution. The microstructure of samples under different compacted conditions obtained from scanning electron microscope demonstrated that montmorillonite content of unit volume and microstructure are the fundamental factors influencing the expansion of bentonite-sand mixture.