Knowing the mechanical behavior of montmorillonite under tensile stress is crucial in earth sciences and geomechanics. However, existing theories and methods are difficult to predict its hydration mechanical properties and inner mechanism within the small layer-spacing. In this paper, through the stress-strain script, tensile molecular dynamics simulation and stress-strain analysis are conducted on montmorillonilte with different hydration amounts to determine the mechanical properties, interaction mechanism and microstructure evolution. It is found that the weakening effect of interlayer hydration on the ultimate stress and tensile modulus is obvious, and the weakening effect is larger in the early stage of hydration; the volume expansion with hydration results from the linear increase in lattice length c. The Z direction tensile modulus is much smaller than the in-plane, that is, the stress has the greatest influence on the mechanical behavior of surface Z direction; after reaching the ultimate tensile stress, the layer separation failure occurs; besides, the interlayer is the main cause of deformation and dominates the tensile mechanical properties of montmorillonite; the tensile stress in the Z direction causes the increase of lattice length c and lattice angle β, while in the X and Y directions, it is mainly the decrease and increase of β. The higher the layer charge density, the denser the bound-water film, the more hydrogen bonds formed, the smaller the volume and lattice length c, and the stronger the tensile mechanical properties.