A quantitative understanding of adsorption and diffusion properties of CH, and H,O in ZIF-8 is crucial for its application in adsorption-hydration coupling CH, storage. Monte Carlo and molecular dynamics simulations are conducted to investigate these properties. Firstly, the spatial distribution of H,O in ZIF-8 cavity and intergranular space is discussed. H,O is mainly distributed in the intergranular space near the outer surface of the cavities. The growth rate of H,O adsorption amount in the intergranular space is about 6 times that in the cavities. Then, the effect of pre-adsorbed H,O on the CH, adsorption in ZIF-8 at 275.15 K and 0–6 MPa is investigated. From the analysis of radial distribution functions and interaction energy, H,O occupies the CH, preferential adsorption sites and reduces effective adsorption volume. Therefore, CH, adsorption capacity decreases with increasing water content. Next, the mutual influence of H,O and CH, on structural properties is analyzed. CH, has no effect on the structure of H,O clusters, while H,O makes CH, together more tightly. Finally, the self-diffusion coefficients of H,O, CH,, and CH,/H,O mixture are calculated. Compared with pure H,O, the presence of CH, enhances the interaction between H,O and ZIF-8 and inhibits the diffusion of H,O. However, compared with pure CH,, the effect of H,O on the diffusion of CH, is more complicated. At a low loading, the presence of H,O inhibits the diffusion of CH, while it facilitates the diffusion of CH, at a high loading. This study provides microscopic insight into adsorption and diffusion in ZIF-8.