Abstract: In this study, montmorillonite was taken as a research target to construct the montmorillonite slit pore model. Meanwhile, Monte Carlo method was used to investigate the adsorption behaviors of methane in montmorillonite pore. On this basis, the influences of different pore sizes, different temperatures, different water contents and different compositions on the adsorption behaviors of methane in montmorillonite pore was explored to reveal the micro-adsorption mechanism of methane in montmorillonite pore. The research results show that the isosteric adsorption heat of methane is less than the 42 kJ/mol and decreases with the increasing of pore size, indicating that the methane adsorption on the montmorillonite can be identified as physic adsorption. With the increasing of pressure or the decreasing of pore size, the adsorption of methane in pores is transferred from the adsorption site with higher energy towards that with lower energy, thus increasing the adsorption capacity of methane. In montmorillonite micro-pores, the adsorption capacity of methane increases with the increase of pore size; while in the meso-pores, it decreases with the increasing of pore size. The proportion of the adsorbed gas amount of methane molecules in montmorillonite pores presents a decline trend as the pressure or pore size increases. When the pore size was more than 6 nm, montmorillonite pores were mainly filled by free gas. With temperature rising, the isosteric adsorption heat of methane is decreased, and the adsorption of methane was transferred from the adsorption site with lower energy towards that with higher energy, thus decreasing the methane adsorption capacity. The water molecules in montmorillonite pores are influenced by the combined effect of Van der Waals force and electrostatic force, and thus can accumulate on pore walls in a directional way. Moreover, these water molecules occupy the adsorption site and space of methane molecules, thus decreasing the methane adsorption capacity. In the multi-composition competitive adsorption, the gas adsorption capacity of montmorillonite is sequenced as follows:carbon dioxide >methane >nitrogen. The increasing of nitrogen or carbon dioxide will cause the reduction in mole fraction of methane in gas phase, the changes in adsorption sites of methane and the decrease of methane adsorption space. The combined effect of three factors leads to a decrease in the methane adsorption capacity.