The development of nuclear energy has brought a great challenge of the efficient treatment of spent radioactive ion exchange resin, and oxidative pyrolysis is a potential method of solving this issue. This study aims to investigate the oxidative degradation and conversion mechanism of tar and off-gas, byproducts produced in the pyrolysis process, in the presence of a catalyst, manganese dioxide. The results show that it presents three processes including liquefaction, gasification and direct gasification in pyrolysis, in which manganese dioxide could reduce the dominant temperature of gasification by 200 ℃. Tar and off-gas undergo secondary pyrolysis, and the yield of tar decreased by approximately 50%. Simultaneously, manganese dioxide could promote the conversion of tar into off-gas, making both of them simplified, for which long-chain hydrocarbons are transformed into short-chain hydrocarbons, CO2, H2O, H2, etc. The catalytic oxidation effect of manganese dioxide is attributed to the adsorption and catalysis of oxygen vacancy, which significantly simplifies the composition of tar and off-gas.