Abstract: To investigate the combustion characteristics and NO_x formation mechanisms of ammonia–hydrogen blended fuels under high-pressure conditions， systematic numerical simulations were performed using Chemkin software based on validated chemical kinetic mechanisms. The results indicate that hydrogen blending not only improves the combustion performance of ammonia but also conditionally regulates NO_x formation. With increasing hydrogen blending ratio， the ignition delay time is significantly reduced， while both the laminar flame speed and adiabatic flame temperature are enhanced. In particular， hydrogen addition markedly extends the ignition limits of ammonia under low-temperature conditions. For all hydrogen blending ratios， the shortest ignition delay and the adiabatic flame temperature peaks are observed at an equivalence ratio of 1.0， whereas the laminar flame speed reaches its maximum at an equivalence ratio of 1.1. Kinetic analysis shows that NH₃ is mainly consumed through dehydrogenation reactions with OH radicals to form NH₂， and HNO is identified as one of the primary precursors for NO formation. Hydrogen blending intensifies the reaction fluxes of key radical pathways， leading to a significant increase in the net formation rate of NO， while the net formation rates of N₂O and NO₂ decrease compared with those of the pure ammonia combustion.