This study primarily investigates the NH₃/H₂/air combustion characteristics within a two-layer porous burner employing a two-dimensional model with a detailed kinetics. The effects of inlet velocity (ug,in) and blending ratio on the combustion characteristics are systematically examined. It is shown that the gas temperature continuously increases from the inlet before entering the reaction zone due to the heat recirculation via porous medium, thereby expanding the stable combustion limit and lean flammability limit of the mixture. Under a hydrogen blending ratio of 20%, the NH₃/H₂ achieves stable combustion in the burner for ug,in=0.2 m/s-0.8 m/s and the equivalence ratio range of 0.5-0.65. The stable flames are stabilized just behind the interface when ug,in<0.9 m/s, then the flames are stabilized towards the burner outlet. The emission of NO increases monotonically with ug,in, ranging from approximately 5320 ppm at ug,in=0.2 m/s to over 18700 ppm at ug,in=0.8 m/s. In contrast, the NO2 emissions remain relatively stable across the range of inlet velocities studied, fluctuating around 100 ppm. The flame tends to be stabilized towards the upstream as the equivalence ratio is increasing from 0.55 to 0.65 under ug,in=0.8 m/s and blending ratio of 0.2, while the NO is increasing linearly from 14310 ppm to 19690 ppm with the equivalence ratio. A preliminary experimental validation is conducted and the same variation trend with the experiment is obtained, but the deviation between the two methods is obvious. This study contributes to enhancing combustion stability, extending flammability limits, and reducing nitrogen oxide emissions, providing key theoretical support for the development of efficient, low-carbon combustion technologies.