Several amine-based ionic liquids (ILs) were synthesized via a one-step method using low-priced organic amines and inorganic acids, and they were mixed with water to form new CO2 absorbents. The effects of the ionic structure, IL concentration, temperature, and pressure on the CO2 absorption performance were investigated. The absorption performance of Its was closely related to the ionic structure, and the CO2 molar absorption capacity in ILs with the same cation followed the order of [NO3] > [BF4] > [SO4] or [HSO4], whereas that with the same anion ranked in the following order: multiple amine > diamine > monoamine. The IL [TETA] [NO3] with 40 wt % concentration showed the best capacity for CO2 absorption. Moreover, low temperature and high pressure favored CO2 absorption. The reaction mechanism of the amine group with CO2 in aqueous solutions of [TETA] [NO3], primary amine, and secondary amine was studied via in situ infrared (IR) spectrophotometry. The results showed that the primary and secondary amines first reacted with CO2 to form carbamate, which decomposed further into bicarbonate with the continuous addition of CO2. However, carbamate generated from the reaction of [TETA] [NO3] with CO2 did not decompose further.