Mixing of Ionic liquids (ILs) changes their physicochemical properties nonlinearly. In this study, we analyzed changes in CO2 absorption abilities of ILs caused by mixing using quantum chemical theory. Based on the conductor-like screening model for realistic solvation (COSMO-RS), we performed comprehensive statistical thermodynamic calculations to evaluate the Henry's law constants of CO2 for 29,754 mixed ILs, which are 1:1 mixtures of [cation][anion(A)] and [cation][anion(B)]. Focusing on the anion effect, it was found that combinations of ILs using a fluorine anion and a nonfluorine-organic anion are effective for increasing CO2 absorption amounts. The improvement of the anion combination can be explained by sigma-profiles, which quantify the polarity of the ion species.