Sodium montmorillonite (Na-MMT) is the predominant active mineral in clays. It is very sensitive to environmental factors, especially changes in temperature and moisture content. However, there is a lack of comprehensive studies on how temperature and moisture content affect the structural properties of Na-MMT on a microscopic scale. In this study, molecular dynamics (MD) simulations were used to analyse the structural and micromechanical behaviour of Na-MMT. The aim is to investigate how different temperatures (from 200 to 700 K) and water contents (10, 20 and 30%) affect the mechanical properties of Na-MMT. The simulation results highlight several key points: (1) The movement and behaviour of water molecules change significantly due to the temperature increase, leading to interlayer swelling, which reduces the mechanical properties. (2) Na-MMT exhibits anisotropy, with mechanical properties in the y-direction being superior to those in the x and z directions with increasing temperature and water content. (3) It was found that the higher the degree of hydration, the worse the mechanical properties of Na-MMT, which is consistent with observations in practical engineering applications. This study deepens the understanding of the microscopic characteristics of Na-MMT and further investigates the microstructural changes associated with the swelling mechanism of bentonite.