In this paper, an anionic functionalized ionic liquid antioxidant [MI][C6H2(OH)(3)COO] was designed and synthesized by a two-step method. The structure was characterized by H-1 NMR and Fourier transform infrared spectroscopy. The mechanism and antioxidant properties of the ionic liquid antioxidants were studied, and the effect on the catalytic oxidation characteristics of iron and copper ions in biodiesel. The results show that [MI][C6H2(OH)(3)COO] can significantly improve the oxidation stability of biodiesel and has a significant inhibitory effect on the catalytic oxidation characteristics of iron ions in biodiesel. DFT calculations show that the BDE of O-H in three phenolic hydroxyl groups in [MI][C6H2(OH)(3)COO] is 300-325 kJ/mol, and that of O-H in [MI][C6H2(OH)(3)COO]-Fe3thorn chelate is 306 kJ/mol. (C) 2020 Elsevier Ltd. All rights reserved.