Abstract: H₂C₂O₄ is widely used in spent fuel reprocessing. For example, in order to recovery plutonium oxalate precipitated mother liquor before returning, it is necessary to reduce the H₂C₂O₄ concentration from 0.1 mol/L to about 10^-4 mol/L. Among the methods of H₂C₂O₄ destroyed in waste liquid, the process of H₂C₂O₄ oxidized in nitric acid solution with Mn²⁺ as a catalyst is the most mature and suitable for industrialization, and its research is very rich. Nitrite plays an important role in destroying oxalic process in the process of H₂C₂O₄ destroyed with Mn²⁺ as a catalyst in nitric acid solution was mentioned in many past studies. However, the specific reaction chain is still ambiguous and controversial on nitrite oxidation of H₂C₂O₄ with Mn²⁺ as a catalyst, and the catalytic reaction mechanism is not clear. To solve these issues in the area of post-processing, the optimization of the treatment of the mother liquor of plutonium oxalate precipitation in the Purex process and the acquisition of H₂C₂O₄ deep destruction technology in spent fuel reprocessing are to be benefited. Firstly, sodium nitrite solutions were continuously added into reaction solutions, at the same time the concentration of H₂C₂O₄ in solutions was determined. H₂C₂O₄ is obviously oxidized in the presence of nitrite and Mn²⁺ in sulfuric acid solutions. Secondly, Mn(Ⅱ) complexed oxalate oxidized to Mn(Ⅲ) with nitrous acid as a oxidant was studied. The results demonstrate that the complexation of Mn²⁺ with H₂C₂O₄ reduces the reaction activation energy of Mn(Ⅱ) oxidized to Mn(Ⅲ), and MnC₂O₄ can be oxidized to Mn(C₂O₄)₃^3- by nitrous acid. Finally, free radicals and substates in reaction process were detected by EPR’s spin capture technique. H₂C₂O₄ is oxidized to ·OOC—COOH with the Mn(Ⅲ) complexed with itself, and Mn(Ⅲ) is reduced to Mn(Ⅱ). The ·OOC—COOH radicals with poor stability rapidly decompose and release reducing substances in acidic solution, and decomposition of H₂C₂O₄ is completed. With the increase of acidity of nitrous acid, the concentration of HNO₃ molecule increases. Thus, the concentration of HNO₃ molecule in the solution can no longer be neglected compared with the concentration of nitrous acid. Therefore, when the concentration of nitric acid is more than 6 mol/L, the decomposition rate of H₂C₂O₄ in the nitric acid solution with Mn²⁺ as a catalyst may be controlled by various reactions, which needs to be further studied.