A decrease in the gas pressure and coal pore size evolution resulting from the comprehensive effects of effective stress increase, coal matrix shrinkage, gas slippage, and Knudsen diffusion impacts gas permeability. This paper focuses on anthracite coal from the Qinshui basin of China to investigate pore size evolution and its impact on CO2 permeability variation. The results show that (a) the mean pore radius of the coal core varies from 7.839 to 42.946 nm under a 0.22.2 MPa mean gas pressure and 2.45.5 MPa confining stress conditions, (b) the decreases in the mean pore radius and gas pressure during pressure depletion under constant effective stress conditions cause the permeability to gradually increase due to gas slippage and Knudsen diffusion, (c) at constant gas pressures, the mean pore radius decrease induced by the effective stress increase primarily leads to a reduction in the permeability, and (d) during pressure depletion under constant confining stress (3.7 and 4.3 MPa) conditions, the permeability changes by multiple factors including the effective stress, matrix shrinkage, gas slippage, and Knudsen diffusion. It is observed that the negative effect from the effective stress is slightly stronger (obviously weaker) than the positive effects from the matrix shrinkage, gas slippage, and Knudsen diffusion at mean gas pressures greater (less) than approximately 0.8 MPa. Finally, an empirical model was established predicting the CO2 permeability change by coupling pore size evolution with the permeability change during gas pressure depletion under constant confining stress conditions. This model comprehensively considers the four effects of effective stress, matrix shrinkage, gas slippage, and Knudsen diffusion and shows consistency with the experimental data.