Abstract: Electron spin resonance (ESR) method was used in researching the conversion of •NH₂ under acidic conditions, the effect of reaction time on the free radical production in HClO₄ system, the effect of pH on the broken bond of N₂H₄, and whether N in HNO₃ impacts on the production of •NH₂. The reaction mechanism of N₂H₄ catalyzed by Pt was determined. The results show that the reaction of •NH₂ captured by DMPO under acidic conditions is •NH⁺₃+HO—H+DMPO=NH⁺₄+DMPO(•OH). In Pt-catalyzed N₂H₄ the HNO₃ doesn’t break bond, and the •NH₂ is only from N₂H₄. In the HClO₄ system, as the reaction proceeds, the trend of N—N bond-breaking in N₂H₄ decreases gradually and the trend of N—H bond-breaking increases gradually. With the increase of pH, the rate of N—N bond-breaking in N₂H₄ decreases rapidly and then increases slowly after pH>3, N—N bond-breaking and N—H bond-breaking coexist in Pt-catalyzed N₂H₄ decomposition process, but the N—N bond-breaking is dominant. In the reaction system, the concentration ratio of H⁺ to N₂H⁺₄ determines the formation rate of •NH₂ from N—N bond breaking, the concentrations of •NH₂ and H^＋ also determine the rate of •NH₂ converted to the products, and the two aspects determine the rate of N₂H₄ decomposition together.