The adsorption and separation properties of 3A zeolite are investigated by using Grand Canonical Monte Carlo (GCMC) simulation. To obtain the adsorption isotherms of water, methyl mercaptan, and methane on 3A zeolite, COMPASS Force Filed is used. The adsorption isotherms of the pure components and the ternary mixtures of water, methyl mercaptan, and methane on 3A zeolites are calculated. The Sips Model is taken into account for the description of water adsorption on 3A zeolite. In addition, the effects of pressure and temperature on the adsorption of components are examined. The results demonstrate that the water has high adsorption selectivity on 3A zeolite, and this kind of adsorbent is a good candidate for the dehydration of natural gas.