In this paper, the adsorption effect of methane (CH4) gas molecular on monolayer PbSe with and without vacancy defects is studied based on first-principles calculations. The effects of the adsorption of methane molecular on monolayer PbSe and on the Se vacancy (VSe) and Pb vacancy (VPb) of monolayer PbSe are also explored. Our results show that methane molecules exhibit a good physical adsorption effect on monolayer PbSe with and without vacancy defects. Moreover, our simulations indicate that the adsorption capacity of CH4molecules on monolayer PbSe can be enhanced by applying strain. However, for the monolayer PbSe with Vse, the adsorption capacity of CH4molecules on the strained system decreases sharply. This indicates that applying strain can promote the dissociation of CH4from VSe. Our results show that the strain can be used as an effective means to regulate the interaction between the substrate material and the methane gas molecules.