A novel absorption method was developed and used for the simultaneous removal of SO2 and NOx whereby triethanolamine (TEA)-modified magnesia was utilized as an absorption slurry and combined with gas-phase oxidation by ozone. TEA was first used as the additive to magnesia slurry to facilitating the aqueous absorption of NOx and SO2. The injection of O-3 increased the solubility of NOx considerably. Furthermore, the addition of TEA facilitated the absorption of NOx in wet combined desulfurization and denitrification processes. In addition, experiments on operating factors included initial pH, TEA concentration, and the effect of SO2 were performed in this study. The results show that after the addition of TEA to a magnesium-based slurry, the absorption efficiency of NO2 improved to almost 100% under alkaline conditions. However, the improvement effect of TEA was less evident in acidic conditions. Based on studies of operating factors and product analysis, the reaction mechanism of TEA-assisted magnesia slurry absorption was proposed. Under alkaline conditions, TEA served as the radical scavenger, terminated the excessive oxidation of sulfite, and then strongly promoted the reaction between NO2- and SO32-. Furthermore, TEA could react with NO2 and SO2 directly, and its alkalinity was beneficial for the absorption of NO2 and SO2. In acidic conditions, TEA reacted with H+ and formed H+-N(CH2CH2OH)(3), and the antioxidant capability of TEA would be lost.