Task-specific ionic liquids (TSILs) have been widely observed to effectively absorb low-concentration SO2 from flue gas by chemical interaction. However, the interaction between SO2 and TSILs is still unclear. The addition of solvents can decrease the viscosity of TSILs and promote mass transfer, but whether the solvents have an impact on absorption is unknown, as is the effect on absorption mechanism. To solve these issues, we synthesized several types of TSILs containing carboxylate anion to capture SO2 from simulated flue gas. The mechanism of absorption of SO2 by the TSILs was investigated in detail using Fourier transform infrared (FT-IR), H-1 nuclear magnetic resonance (NMR), and C-13 NMR The results show that chemical interactions can be found between the carboxylate anions of TSILs and SO2. Simultaneously, the effect of several solvents on SO2 absorption capacity of TSILs was studied. The results indicate that ethylene glycol (EG) in TSILs has a strong influence on SO2 absorption by guanidinium- and alkanolaminium-based TSILs but for quaternary ammonium-based TSILs, EG has no effect. Finally, the interaction between EG and TSILs was investigated by FT-IR, and the absorption mechanism was studied. It has been found that the addition of EG can improve the basicity of guanidinium- and alkandlaminium-based TSILs to increase the absorption capacity.