A new strategy involving the computer-assisted design of substituted imidazolate-based ionic liquids (ILs) through tuning the absorption enthalpy as well as the basicity of the ILs to improve SO2 capture, CO2 capture, and SO2 /CO2 selectivity was explored. The best substituted imidazolate-based ILs as absorbents for different applications were first predicted. During absorption, high SO2 capacities up to ≈5.3 and 2.4 molSO2  molIL-1 could be achieved by ILs with the methylimidazolate anions under 1.0 and 0.1 bar (1 bar=0.1 MPa), respectively, through tuning multiple N⋅⋅⋅S interactions between SO2 and the N atoms in the imidazolate anion with different substituents. In addition, CO2 capture by the imidazolate-based ILs could also be easily tuned through changing the substituents of the ILs, and 4-bromoimidazolate IL showed a high CO2 capacity but a low absorption enthalpy. Furthermore, a high selectivity for SO2 /CO2 could be reached by IL with 4,5-dicyanoimidazolate anion owing to its high SO2 capacity but low CO2 capacity. The results put forward in this work are in good agreement with the predictions. Quantum-chemical calculations and FTIR and NMR spectroscopy analysis methods were used to discuss the SO2 and CO2 absorption mechanisms.