Abstract: This study investigates the hydration swelling of montmorillonite and the evolution of its mechanical properties during compression and tension through molecular dynamics simulations. By quantitatively analyzing key parameters such as basal spacing, water molecule distribution density, interaction energy, hydration energy, and intrusion energy, this study reveals the influence of water content on the structural stability and mechanical anisotropy of montmorillonite interlayers. The results show that as the water content increases, the basal spacing of montmorillonite expands. The infiltration of water molecules weakens the van der Waals forces and enhances electrostatic interactions, leading to an increase in interlayer spacing and causing swelling. The simulation results are consistent with experimental data, confirming the reliability of the simulation method. It is also found that montmorillonite crystals exhibit significant anisotropy in their mechanical behavior under compression and tension. Increased water content notably weakens the mechanical properties of montmorillonite, particularly in the X and Y directions, where strain concentration and shear failure are evident. In contrast, the Z direction demonstrates higher toughness. The breaking and reformation of bonds such as Si-Ob, Al-Ob, Al-Oh, and Oh-Ho dominate the mechanical response process of the crystal. During tension, the Si-Ob bonds are the most prone to breakage, while during compression, a synergistic effect of bond breaking and reformation occurs. Notably, structural reorganization of the interlayers under compression in the Z direction is particularly pronounced.