Novel kinds of environmentally benign and stable absorbents, quaternary ammonium inner salts betaine (Bet) and L-carnitine (L-car) in aqueous solution, were designed and applied to efficiently and reversibly capture sulfur dioxide (SO2) in flue gas. The effects of the inner salt concentration, absorption temperature, SO2 concentration, and reuse cycle on the absorption of SO2 by the two absorbents were investigated. The results show that SO2 absorption capacities of the absorbents with 50% inner salt mass fraction were 0.155 mol of SO2/mol of Bet for Bet aqueous solutions and 0.599 mol of SO2/mol of L-car for L-car aqueous solutions, at 40 degrees C with a SO2 concentration of 2%, and the absorption capacities of absorbents did not change after 5 absorption/desorption cycles. Furthermore, Fourier transform infrared spectroscopy and proton nuclear magnetic resonance results demonstrate that the SO2 absorption follows the law that a strong acid (H2SO3) replaces a weak acid (COOH). The absorbents could be used for more than 1 week without any change in the structure, indicating the promise for industrial applications in desulfurization of flue gas.