Abstract: Ammonia-enriched oxy-fuel combustion is a potential zero-carbon and high-efficiency power generation technology, but the high flame temperature and NO_x emissions limit its development. Steam dilution can effectively suppress the ammonia-enriched oxy-fuel flame temperature and NO_x emissions, and the mechanism of the dilution effect needs to be investigated to balance the combustion efficiency and flame characteristics. This study conducted kinetic simulations of NH₃/O₂/H₂O flames under near-realistic gas turbine inlet temperature and combustor pressure conditions to investigate the effects of steam on the NH₃/O₂/H₂O flame at varying equivalence ratios and pressures, and the physical and chemical effects of steam dilution in ammonia-enriched oxy-fuel combustion were decoupled. The results show that the NH₃ conversion rate of NH₃/O₂/H₂O flame is close to 1, and NO accounts for more than 99.99% of NO_x emission. Steam dilution and pressurization inhibit and promote the flame temperature, respectively. Under high-pressure conditions (1.0 MPa), a dilution ratio of 0.25 can reduce the laminar flame speed and flame temperature by 102.89 cm/s and 376.23 K, respectively. At fuel-lean conditions, NO emissions first decrease and then increase with pressure, while pressurization has a monotonic inhibition on NO emissions at fuel-rich conditions. A dilution ratio of 0.25 can reduce the volume fraction of NO emissions to 1.647×10⁻⁴ (15% O₂ basis) at 1.0 MPa. Compared to the chemical effects, the physical effects of steam dilution have more significant effects on the laminar flame speed, flame temperature, and NO_x emissions of NH₃/O₂/H₂O combustion. Among the physical effects, the dilution effect is dominant, substantially suppressing the laminar flame speed and temperature while promoting NO_x emissions. The thermodynamic, transport, and radiation effects also reduce the flame speed and temperature, with the radiation effect being the smallest. As the pressure increases, the physical effects of steam dilution become more dominant while the chemical effects are weakened. In the chemical effects of steam dilution, the direct reaction effect is more significant than the third-body effect under pressurized conditions, and both suppress the flame speed and flame temperature. The direct reaction effect reduces NO emissions, while the third-body effect does the opposite.