Grand canonical Monte Carlo and canonical ensemble molecular dynamics simulations were used to investigate water adsorption properties in mesoporous silica thin films. The effect of pore radius on the adsorption properties was assessed using two models of mesoporous silica thin films having different pore radius and film thickness (1.38 and 5.66 nm in Model 1, respectively, and 1.81 and 7.30 nm in Model 2, respectively). In the simulations, a water adsorption layer or water menisci were formed in a mesopore accompanying the growth or shrinkage of stable adsorption layers on the upper and lower surfaces. The thickness of a water adsorption layer immediately before the onset of capillary condensation and the curvature radius of a water meniscus prior to capillary evaporation were identified. In the initial stage of layer adsorption, the surface state and radius of the pore affected the coordination structure of water around silanol groups. In the pore-filling state, the hydrogen bond network of water in the pore was similar but not the same as that in the bulk liquid; the difference was because the pore wall affected the position of water molecules. This difference may confer unique dynamic properties upon the water in mesoporous silica.