Abstract: Based on molecular dynamics and Metadynamics, the matric potential of the sodium montmorillonite and calcium montmorillonite at low moisture content conditions are discussed. And the soil water characteristic curve is established at a moisture content range of 5% - 30%. The matric suction of sodium montmorillonite and calcium montmorillonite are found to decline exponentially with the increase of moisture content. When the moisture content is less than 20%, the matric suction of sodium montmorillonite is slightly less than calcium montmorillonite. When moisture content is more than 20%, the matric suction of sodium montmorillonite is slightly larger than that of calcium montmorillonite. The matric suction of sodium montmorillonite is about 4.2 GPa at the moisture content of 5%, and the matric suction of calcium montmorillonite is about 4.4 GPa. These methods and results provide references for suction measurements by experiment or predicted values by theoretical models.