Oxalic acid is an intermediate product of a variety of organic matters degraded in wastewater, it is difficult to remove directly by oxidization because of its low reaction rate with ozone. A series of Manganese nitride carbon catalysts with different loadings were prepared using different molar ratios of melamine and manganese(Ⅲ) acetate dehydrate through the pyrolysis calcination. The degradation effects of oxalic acid by catalytic ozone with MnOx/g-C3N4 on different conditions were studied. The effects of the molar ratio of melamine and manganese(Ⅲ) acetate dihydrate, catalyst dosage, the pH of solution, ozone concentration and TBA on degradation of oxalic acid by catalytic ozone were investigated. The results showed that, compared with ozone alone, the removal rate of oxalic acid improved greatly by catalytic ozone with MnOx/g-C3N4. The removal rate of TOC was 89.1% in 60 minute when 0.15g/L of catalytic was added, which is prepared with the mole ratio of melamine and Manganese(Ⅲ) acetate dehydrate being 20∶[KG-*3]1. The removal rate of oxalic acid decreased slightly when tert butyl alcohol was added into aqueous solution, which proved that ·OH was involved in the process of catalytic ozonation of oxalic acid but it was not the only factor to make oxalic acid degraded.