As an important component of natural gas hydrate reservoir, montmorillonite is of great research significance to the exploitation and application of natural gas hydrate. The inhibition of salt ions in seawater on the hydration expansion of montmorillonite has important effects on the distribution of free water and bound water in hydrate reservoirs. Water content in montmorillonite with NaCl solution was measured by thermogravimetric method combined with several pretreatment methods (the centrifugal method, the direct drying method and the freeze-dried method). The effects of salt concentration on bound water and free water content of montmorillonite were analyzed. The variations of total water content and bound water content in montmorillonite with different NaCl solution concentration were discussed. The hydration and expansion characteristics of montmorillonite and the inhibition of salt ions were studied. The results indicated that the hydration expansion of montmorillonite was inhibited by salt ions. The existence of salt ions reduced osmotic hydration force. Meanwhile, it affected the van der Waals force between water molecules, the hydrogen bonding force and the electrostatic attraction force between water molecules and the surface of montmorillonite. The total water content of montmorillonite decreased with the increase of salt ions concentration. In the same time, bound water content of montmorillonite was inverse related to the salt ions concentration, and approached the minimum when the salt ions concentration was 0.2 mol/L, as the inhibition effect of salt ions on the hydration expansion of montmorillonite reached the peak. When the ion concentration was greater than 0.2 mol/L, the content of free water and bound water did not change significantly with the increase of salt ion concentration. Therefore, the study on the influence of salt ions on hydration and expansion characteristics of montmorillonite, so as to further explore the influence of salt ions on the formation of hydrate in montmorillonite, has important guiding significance for the study of hydrate distribution characteristics in deep-sea sediments.