The removal of NOx (approximately 90% of which is NO) from flue gas is a crucial process for clean power generation from coal combustion. Oxidation of NO to NO2 followed by NO2 absorption using sorbents is considered to be a promising technology alternative to selective catalytic reduction (SCR). This study investigated the absorption of NO2 in flue gas by ethylene glycol (EG)-tetrabutylammonium bromide (TBAB) deep eutectic solvents (DESs) under a range of experimental conditions. The effects of experimental conditions including molar ratio of EG to TBAB, operating temperature, residence time, and the O2 and steam partial pressure in the flue gas on the denitrification performance of EG-TBAB DESs were systematically analyzed. The concentrations of NO2 in the inlet and outlet were evaluated using a flue gas analyzer. The chemical structure changes of DESs after denitrification were characterized using Fourier transform infrared (FT-IR) spectroscopy. The obtained analysis signified that maximum denitrification efficiency and capacity were achieved at a EG/TBAB molar ratio of 5:1, 50 °C, and 6 s residence time. EG-TBAB DESs were able to maintain a stable denitrification performance after five absorption-desorption cycles. The results of quantum chemical calculation and 1H NMR spectra of EG-TBAB DES show that bromide anions in the EG-TBAB DES maintained strong interactions with NO2 via hydrogen bonding, leading to increased NO2 adsorption. The presence of O2 and steam in the flue gas improved the absorption of NO2 in EG-TBAB DESs due to chemical reactions and formation of nitrate.