In this work, the CO2 absorption capacity of superbase derived ionic liquid, i.e., 1,8-diazabicyclo [5,4,0] undec-7-ene imidazole ([HDBU][IM]) mixed with three different cosolvents (dimethyl ethers of polyethylene glycol (DEPG), propylene carbonate, and ethylene glycol) was studied, and [HDBU][IM]-DEPG was selected for further investigation, showing [HDBU][IM]-DEPG with 2:1 mass ratio at the temperature of 298.15 K exhibits the optimal performance of CO2 absorption capacity and viscosity. The gas solubilities for CH₄, N₂, and two gas mixtures (75% N₂ + 25% CO₂; 60% CH₄ + 40% CO₂) were studied, indicating the selectivity can be up to 17, and the real selectivity is higher than the ideal one. Thermodynamic modelling was carried out, and the species distributions, as well as the physical and chemical contributions, were analyzed, illustrating the reliability of the thermodynamic model with an average relative deviation lower than 2.37%. The decreased DEPG content and increased temperature favor chemisorption, contributing more than 90% when the pressure was lower than 0.4 MPa, while with increasing pressure, the physical contribution gradually increases up to 30%. This work evidences that [HDBU][IM]-DEPG is an excellent candidate for CO₂ separation due to its high CO₂ absorption capacity and low viscosity, and the high CO₂ selectivity over CH₄ and N₂ expands its capability for real industrial applications.