Abstract: This study investigates the hydration swelling of montmorillonite and the evolution of its mechanical properties through molecular dynamics simulations, revealing the influence of water content on structural stability and mechanical anisotropy. The results show that as water content increases, the basal spacing of montmorillonite expands. The infiltration of water molecules weakens van der Waals forces and enhances electrostatic interactions, leading to crystal swelling. Water molecule distribution transitions from concentrated adsorption at low water content to multi-layer uniform distribution at high water content. The simulation results are consistent with experimental data, confirming the reliability of the method. It is found that montmorillonite exhibits significant anisotropy in mechanical behavior: X and Y directions show brittle failure, while the Z direction demonstrates higher toughness. Increased water content notably weakens mechanical properties, particularly manifesting as strain concentration and shear failure in X and Y directions. Microscopic analysis reveals that the breaking and reformation of Si―Ob, Al―Ob, Al―Oh, and Oh―Ho bonds dominate the mechanical response process. During tension, Si―Ob bonds are most prone to breakage, while during compression, a synergistic effect of bond breaking and reformation occurs, with interlayer structural reorganization being particularly significant under Z-direction compression.