Utilization of cheap and natural resources is an important topic in green chemistry. A new class of betaine-based aprotic task-specific ionic liquids (ILs), including [C₄bet][TFSI], [C₄bet][DCA], [C₄bet][SCN], [1O2bet][TFSI], [1O2bet][DCA], [2O2bet][TFSI], and [2O2bet][DCA], has been designed and prepared through a new ionic liquidization strategy from the renewable natural resource betaine. Their physicochemical characteristics, including spectral properties, density, viscosity, melting point, glass transition temperature, thermal stability, conductivity and electrochemical window, have been comprehensively studied. They were first applied as new absorbents for SO₂ capture, and [C₄bet][SCN] exhibited the maximum absorption capacity of 0.93 mol_SO2/mol_ILs at 20 °C and 1 atm (SO₂/N₂ = 10% vol.) with rapid absorption rates in 21 min. Moreover, the effect of temperature, pressure, and water content on the absorption performance of SO₂ was investigated. The possible absorption mechanism was studied using FT-IR, NMR and quantum chemical calculation. In addition, the [C₄bet][SCN] can maintain the high absorption capacity and the rapid absorption rates for 25 absorption/desorption cycles.