In order to study the mechanism of methane hydrate formation in porous media, the formation experiments of methane hydrate in porous media at the constant pressure were performed in the temperature range of 274.15-276.15 K and the pressure range of 5-8 MPa. The silica gels with the average pore diameters of 129.5, 179.6, and 332 angstrom were used as the porous media for the experiments. The experimental results indicate that the final gas consumption increases with the increase of the formation pressure and the decrease of the formation temperature. Based on the shrinking core model, the reaction-controlled kinetic model and the diffusion-controlled kinetic model for hydrate formation in silica gels were built, respectively. The reaction-controlled kinetic model well fits the kinetic data in 129.5 angstrom and 179.6 angstrom silica gels, and the diffusion-controlled model well fits the kinetic data in 332 angstrom silica gels with a relatively high regression coefficient (R-2 > 0.99). The formation rate of the methane hydrate is controlled by the gas diffusion process in 129.5 angstrom and 179.6 angstrom silica gels, and is controlled by the reaction process in 332 angstrom silica gels.