Surface wettability of swelling clay minerals plays an important role in the flocculation-dispersion, sorption, flotation and interfacial interactions behaviors. But it is difficult to determine, not only for the small particle size, but also for the abilities of hydration and swelling. In this work, surface wettability of montmorillonite (0 0 1) surfaces as affected by surface charge and exchangeable cations has been investigated through molecular dynamic (MD) simulations. The adsorption energy between montmorillonite (0 0 1) surface and water molecules was calculated to qualitatively characterize the wettability behaviors, and more negative adsorption energy means stronger affinity between montmorillonite surface and water molecules. It has been found that the negative charged montmorillonite surface has very weak adsorption energy for water molecules but very strong electrostatic attraction for exchangeable cations. Besides, these exchangeable cations have very strong hydration ability thus to acquire several layers of hydration shells around the cations, the effect of which is to impart an overall hydrophilic nature to the clay mineral. As a result, exchangeable cations are being adsorbed on montmorillonite (0 0 1) surface in the form of ionic hydration shell. And it is the exchangeable cations that have much stronger affinity to water molecules than montmorillonite surface, thus the adsorbed exchangeable cations on montmorillonite surface play the dominant role in determining the surface wettability behaviors.