Simultaneous capture of SO₂ and NO _x from flue gas is critical for coal-fired power generation. In this study, environmentally friendly and high-performance deep eutectic solvents based on ethylene glycol and ammonium bromide were designed to capture SO₂ and NO₂ simultaneously. The SO₂ and NO₂ absorption performances and absorption mechanisms were systematically investigated by ¹H NMR and Fourier transform infrared (FT-IR) spectroscopy in combination with ab initio calculations using Gaussian software. The results showed that EG-TBAB DESs can absorb low concentrations of SO₂ and NO₂ from the flue gas simultaneously at low temperatures (≤50 °C). ¹H NMR, FT-IR, and simulation results indicate that SO₂ and NO₂ are absorbed by forming EG-TBAB-SO₂-NO₂ complexes, Br^- is the main active site for NO₂ absorption, and NO₂ is more active in an EG-TBAB-NO₂-SO₂ complex than SO₂. EG-TBAB DESs exhibit outstanding regeneration capability, and absorption capacities remain unchanged after five absorption-desorption cycles. The fundamental understanding of simultaneous capture of SO₂ and NO₂ from this study enables DES structures to be rationally designed for efficient and low-cost desulfurization and denitrification reagents.