Abstract: Partial pre-decomposition of ammonia is an effective means to improve the combustion performance of ammonia. This work conducts numerical simulation on an ammonia diffusion combustion device based on ammonia pre-decomposition strategy. The effects of ammonia pre-decomposition ratio, equivalence ratio, and residual heat from ammonia pre-decomposition on combustion and pollutant emission characteristics are analyzed. As the ammonia pre-decomposition ratio increases, the flame temperature increases, the high-temperature zone of the flame moves closer to the nozzle, and the NO_x emissions decrease. NO emission dominates in NO_x emissions, increasing first and then decreasing as the equivalence ratio increases. N₂O emission mainly occurs under low ammonia pre-decomposition ratio and low equivalence ratio conditions, and hardly exists after the ammonia pre-decomposition ratio reaches over 50%. Preheating fuel with residual heat from ammonia decomposition can stabilize combustion and significantly decrease ammonia leakage without increasing NO_x emissions when the ammonia pre-decomposition ratio does not exceed 30%. This study shows that increasing the ammonia pre-decomposition ratio and equivalence ratio can effectively improve the combustion and pollutant emission characteristics of ammonia, and the residual heat from ammonia decomposition should be fully utilized under small proportion ammonia pre-decomposition conditions.