The production of formic acid and acetic acid by hydrothermal oxidation of alkali lignin, a kind of biomasses, was investigated using a batch reactor with H2O2 oxidant. Experiments were performed over a wide range of conditions with temperature varying from 260 to 320 °C, oxygen supply varying from 60% to 120%, and reaction time varying from 30 to 150 s. The highest yield of formic acid was 4.9% at 280 °C for 120 s with the additive ratio of H2O2 100%. The highest value of acetic acid was 12.3% at 300 °C for 120 s with the additive ratio of H2O 2 100%. Based on the intermediate products identified by GC/MS and HPLC, reaction pathways of alkali lignin are discussed. It was found that maleic acid and fumaric acid were two primary unsaturated intermediate products. The production of formic acid and acetic acid were come from the oxidative decomposition of intermediate products in the oxidation process. Increasing the formation of saturated dicarboxylic acids and glutaconic acid would enhance the acetic acid yield.