In this study, we used tetraethylammonium chloride (TEAC), diethanolamine (DEA), and N-methyldiethanolamine (MDEA) to prepare ternary DES and binary DES to absorb CO2. We found that their formation was due to the hydrogen bond interaction between hydrogen bond acceptor and hydrogen bond donor (HBD). Surprisingly, TEAC/MDEA/DEA can react with CO2, but TEAC/MDEA cannot react with CO2. Unexpectedly, after adding DEA to TEAC/MDEA, the ternary TEAC/MDEA/DEA DES can react with CO2. Nuclear magnetic resonance spectroscopy and Fourier infrared spectroscopy results showed that the accidental CO2 absorption behavior mainly depended on the HBD DEA, because the imine group in DEA reacted with CO2 to form carbamate, thereby absorbing CO2, while the hydroxyl group on MDEA and the hydroxyl group of DEA did not interact with CO2. Through thermal stability analysis, TEAC/MDEA/DEA system with the molar ratio of 1:3:4 is more stable. We further studied the influence of molar ratio, temperature, water content, and other factors on the absorption of CO2 by ternary DES. In addition, TEAC/MDEA/DEA (1:3:4) was regenerated at 80°C, and the absorption capacity of DES was almost unchanged after five absorption–desorption cycles.