Abstract: The radiolysis of near-field groundwater in high-level radioactive waste(HLW) repository will produce H₂O₂ which is catalytically decomposed by fission product alloy particles(ε-particle) belongs to the surface interface process. In this work, palladium powder was used to simulate ε-particles. The effects of total pressure and partial pressure of hydrogen on H₂O₂ decomposition reaction were studied by autoclave experiments. The reaction rate constant is derived by the first-order kinetic model fitting. The formation of detectable HO· in the reaction process without hydrogen was proved by experiments with adding capture agent and quencher of HO·.The catalytic activity and morphological changes of palladium particles as a function of time were investigated. It is inferred that the product oxygen adsorbed on the surface of palladium has toxic effect. By continuously monitoring the change of H2O2 concentration in the filtrate, it was detected that the decomposition of H₂O₂ in the filtrate is similar to Haber-Weiss reaction in reaction process. The influence mechanism of palladium particles on the decomposition of H₂O₂ is deduced, and the results of this work can provide significant reference data for the safety assessment of the repository.