Based on the two mechanisms of 3, 7-dinitro-1, 3, 5, 7-tetraazabicyclo[3,3,1] nonane (DPT) nitrolysis via HNO₃-NH₄NO₃ or HNO₃, the effects of NH₄⁺ in nitrolysis of DPT were studied by experiment and theoretical calculation. The effect of NH₄⁺ on the yield of HMX was studied with adding the ammonium salts including (NH₄)₂HPO₄, (NH₄)₂SO₄ and CH₃COONH₄ to nitrate solution system. Results show that the above-mentioned ammonium salts can improve the yield of HMX to 41.5%, 37.4% and 20.7%, respectively. Their function is similar to NH₄NO₃. In different nitric acid-ammonium salt systems, when the molar ratio of NH₄⁺ and DPT is close to 10, the yield of HMX reaches a maximum value of 56.3%, 52.2% and 35.5%, respectively. Results of the nitrolysis of DPT in nitric acid-ammonium salt systems and nitric acid-nitrate system are compared, finding that NH₄⁺ plays a dominant role in improving the yield of HMX. The reaction mechanism of NH₄⁺ in the nitrolysis process of DPT is theoretically explained by the density functional theory (DFT), deriving that the activation energy of DPT nitrolysis in HNO₃/NH₄⁺ is 133.95 kJ·mol^-1, lower than 376.73 kJ·mol^-1 in HNO₃.