Knowing the mechanical behavior of montmorillonite (MMT) under tensile stress is crucial in earth science 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 (MD) simulation and stress-strain analysis are conducted on MMT 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 ultimate stress and tensile modulus is obvious, and the weakening effect is greater 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; when the ultimate stress is reached, the layer separation failure occurs; besides, interlayer is the main cause of deformation and dominates the tensile mechanical properties of MMT; the tensile stress in 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. This work quantitatively reveals the basic mechanical properties and internal structure mechanism of MMT under different hydration, tensile stress, and layer charge density, laying a foundation for macro (micro) control of geotechnical disasters and evaluation of soil engineering performance.