Carboxylic acids (CAs) are considered as an important group of chemicals and are widely used in the chemical industry. The production of CAs via oxidation of lignite is a promising industrial process. In the present work, we found that H5PV2Mo10O40 (HPA) was beneficial to catalytic oxidation of lignite by O-2 in aqueous H2SO4 solutions to produce CAs. The effects of HPA concentration, H2SO4 concentration, reaction temperature, initial O-2 pressure, and reaction time on lignite conversion and CA yield were studied. The catalytic oxidation of lignite in aqueous HPA/H2SO4 solutions with 02 can yield 56.9 wt % CAs, including 33.5 wt % formic acid, 14.4 wt % acetic acid, 1.5 wt % succinic acid, 1.1 wt % oxalic acid, and 6.4 wt % benzene carboxylic acids (BCAs, including 12 types) at 170 degrees C and 3 MPa for 60 min. In this catalyst system, the existence of H2SO4, can change the catalytic activity of HPA, and the synergistic effect of HPA and H2SO4 can significantly promote the production of CAs. V-v can oxidize lignite or intermediates to form V-IV, which can be reoxidized by O-2 to complete a redox cycle. In the catalytic oxidation, lignite was converted into water-soluble intermediates at first, and then the water-soluble intermediates were converted to CAs. The catalyst system was reused four times without significant decline in activity. Compared with the traditional alkali-oxygen oxidation, this method can decrease the usage of mineral acid and alkali and lower reaction temperatures.