Abstract: Because of its particularity, bentonite has been studied by many scholars. In the actual project, it causes many problems. Explores the expansive property of Nanning bentonite, through X-ray diffraction test to determine the mineral composition of Nanning bentonite, to determine the influence of montmorillonite on swelling. Combined with different cationic montmorillonite flocculation test, it was found that the concentration of sodium ions increased. The flocculation of Montmorillonite is enhanced and the critical value of magnesium ions for flocculation of Montmorillonite is rather than the critical concentration. The colloid is stabilized by the zeta potential, which is caused by the exclusive function, and the expansion mechanism is analyzed. The influence of cation on the swelling rate of Nanning Bentonite is studied based on the free expansion test. It was found that with the increase of (Mg) cation concentration, the swelling rate of Nanning bentonite decreased, and the time of stability experienced in the expansion experiment was increased with the increase of cationic valence. The results of low cation concentration and pure water test were similar. The ionic type had an effect on the volume expansion of Montmorillonite. At the same concentration increased, the order of volume expansion is NaClMgCl2.