The development of advanced adsorbents with high CO2 capture and separation performances is essential for decreasing atmospheric CO2 concentration. Herein, grand canonical Monte Carlo and density functional theory were employed to investigate the adsorption and separation properties of CO2, N2, and CH4 in 2D covalent organic framework (COFs) anchored by alkali metal (AMs) Li, Na, and K (UPC-nAMs, n = 3 or 6) at 298 K and 0–1.0 bar. The anchoring of AMs formed favorable adsorption sites, endowing UPC-nAMs with better CO2 capture and separation performances. This effect was more evident with an increase in Li/Na/K concentration. The CO2 adsorption capacity of UPC-6Li/Na/K was ∼ 221.64, 188.28, and 140.50 cm3 cm−3. The smaller atomic size of Li occupied less adsorption space, resulting in the highest CO2 capture capacity of 221.64 cm3 cm−3 with selectivity of 308/206 over N2/CH4 in UPC-6Li at 298 K and 1.0 bar. Structural stability, pore characteristics, isothermal adsorption heat, van der Waals and Coulomb interactions, gas distribution density, adsorption configuration, and gas radial distribution function were analyzed to identify the intrinsic mechanism of anchoring Li/Na/K to improve CO2 capture and separation performances.