Understanding the adsorption mechanism of CO2/CH4 in kaolinite clay is essential for the carbon dioxide geological sequestration and enhanced gas recovery in shale reservoirs. In the present work, grand canonical Monte Carlo simulations were employed to investigate the mechanism of competitive adsorption of CO2/CH4 in kaolinite clay. The effects of pore size (1-6 nm), pressure (0.1-30 MPa), temperature (298-378 K), and moisture content (0-0.122 g/cm(3)) on the adsorption behaviors of pure CH4 and CO2/CH4 mixture were explored in-depth. Specifically, two adsorption layers, i.e., strong and weak adsorption layers, in kaolinite slitlike micropore under high pressure condition have been observed. It was found that pore size and pressure have great effects on the gas adsorption mechanism in kaolinite. The two adsorption mechanisms including monolayer adsorption and micropore filling under high pressure or small pore size conditions were discussed. In addition, simulation results showed that CO, has much stronger adsorption ability than CH4 in kaolinite. The adsorption capacity of CH4 was significantly suppressed in the presence of CO,, especially in the strong adsorption layer. An adsorption selectivity over 7 has been found in the strong adsorption layer. Temperature and moisture content have great influences on the adsorption capacity and adsorption selectivity. However, the influences have different scales in strong and weak adsorption layers. It is expected the obtained results could provide insights into the adsorption mechanism of CO2/CH4 and offer fundamental data for a CO2 sequestration and enhanced gas recovery (CS-EGR) project in kaolinite clay.