Traditional amine solutions for CO2 capture are associated with many defects such as sacrificed CO2 capacities and high volatility. Herein, a new type of mixed solvent was constructed by diluting polyethylenimine (PEI) in tetrabutylphosphonium 2-fluorophenolate ([P-4444][2-F-PhO]). PEI has rich amine groups and strong chemical reactivity to CO2. [P-4444][2-F-PhO] also enables the reversible chemical reaction with CO2; however, it has relatively small viscosity and does not present considerable change in viscosity after loaded with CO2. The CO2 capture performance of mixed solvents was examined systematically. It is demonstrated that the dissolution of CO2 in PEI can be significantly sped up with the assistance of [P-4444][2-F-PhO], and considerably high CO2 capacities (2.62 mol/kg at 313.2 K and 15.0 kPa) are achieved in the mixed solvents. Further considering the almost nonvolatile properties of PEI and [P-4444][2-F-PhO], the mixed solvents are regarded as promising absorbents for capturing CO2 from industrial streams.