To confirm the rules and transformation conditions of shale gas adsorption and establish a model for evaluating the adsorption capacity of shale gas quantitatively, it is necessary to reveal the shale gas adsorption mechanism. The adsorption mechanism of CH4 and CO2 in Kaolinite slit nanopores has been studied under the simulated conditions of 90 °C and 30 or 50 MPa by the grand canonical Monte Carlo (GCMC) method. The results indicate that CH4 is controlled only by the Van der Waals forces on the mineral surface because CH4 is nonpolar, while CO2 is controlled by both Van der Waals forces and Coulomb forces due to a certain electric quadrupole moment, which makes the adsorption capacity of CO2 on kaolinite greater than that of CH4. Due to the overlapping adsorption potential on the kaolinite surface of micropores (1 nm), the peak of the density profile is higher in the micropores than the peak in the mesopores (4 nm), resulting in the filling effect in the micropores. On the surface of the silicon-oxygen octahedron, the adsorption site for CH4 and CO2 is in the center of the silicone hexagon-ring, and CO2 with a quadrupole moment shifts near the polar oxygen atoms. In contrast, the adsorption sites of CH4 are relatively dispersed on the surface of the aluminum-oxygen octahedron with a hydroxyl group, while the adsorption sites of CO2 are concentrated in the location of the aggregated oxygen atoms. When CH4 and CO2 coexist, CO2 tends to be adsorbed prior to CH4. With the proportion of CO2 increasing, the competitive adsorption effect is gradually aggravated, which suggests the rationality of injecting CO2 to improve the recovery efficiency of shale gas. These findings can provide theoretical support for shale gas exploration and development.