Abstract: 【Objective】Auto-ignition induced by high-pressure hydrogen leakage is one of the major safety concerns in hydrogen utilization. Ammonia blending has been regarded as a promising suppression strategy, while release pressure is a key parameter affecting auto-ignition behavior.【Methods】In this study, Chemkin-Pro coupled with the Otomo mechanism was employed to investigate the chemical kinetic effects of release pressure on the auto-ignition of NH₃/H₂/air mixtures. Simulations were conducted at release pressures of 6–12 MPa, initial temperatures of 1000–1600 K, an equivalence ratio of 1, and a fixed ammonia blending ratio of 5%. The ignition delay time, sensitivity of key elementary reactions, radical concentrations, production rates, and reaction pathways were systematically analyzed.【Results】The ignition delay time decreased markedly with increasing pressure. When the pressure increased from 6 MPa to 12 MPa, the ignition delay time at 1000 K decreased by approximately 44.2%, indicating that pressure promotes ignition within the investigated conditions. Sensitivity analysis showed that the chain-branching reaction H + O₂ = O + OH (R1) exerted the strongest promoting effect, whereas the third-body reaction H + O₂ (+M) = HO₂ (+M) (R13) inhibited ignition. Although the absolute values of the sensitivity coefficients of key reactions increased with pressure, the overall contribution of the dominant promoting reactions remained stronger, leading to enhanced system reactivity. Further analysis revealed that, as the pressure increased, the mole fractions of H, O, and OH increased by about 47.72%, 66.33%, and 17.05%, respectively, whereas that of HO₂ decreased by about 41.40%, suggesting that the system became more favorable to rapid chain branching. Higher pressure also accelerated chain-branching and chain-propagation reactions in the H/O sub-mechanism. In particular, HO₂ was rapidly converted into OH through reactions such as HO₂ + H = 2OH (R15), thereby promoting radical accumulation during the early stage of ignition. Reaction-path analysis further showed that the contribution of OH increased in most nitrogen-containing pathways, whereas the relative contributions of H and O decreased, indicating that ignition progression became more dependent on OH-dominated oxidation pathways at elevated pressures.【Conclusion】Increasing release pressure enhances the reactivity of the H/O sub-mechanism, promotes the accumulation of active radicals, and shifts radical distribution toward pathways favorable for ignition, thereby significantly shortening the ignition delay time and increasing the auto-ignition tendency of ammonia-hydrogen blended fuels. These results provide theoretical support for the risk assessment of high-pressure hydrogen leakage and for the optimization of ammonia-based suppression strategies.