Two-stage pure ammonia combustion significantly improves combustion efficiency while reducing NO_x emissions. However, at the primary stage, the low temperature and poor fuel–air mixing result in flame instability. An autothermal recirculating combustor (ARC) is proposed, and it is used as the primary-stage combustor under fuel-rich conditions (ER > 1) in the study. The mixing, preheating and reaction characteristics of NH₃ and air in ARC were investigated by numerical simulation. The low-temperature NH₃ and air introduced are rapidly preheated and efficiently mixed, this widens the upper limit of the combustion equivalence ratio, and stable flame can be maintained even at an equivalence ratio of 1.83. In the 1020–1350 K regime, both oxidation and decomposition of NH₃ proceed rapidly. Hydrogen radicals in flame act as a key chain carrier, enabling OH radical production and NH₃ dehydrogenation, thus controlling the reaction kinetics at ER > 1. With the increase of equivalence ratio from 1.30 to 1.66, H₂ concentration and conversion rate at ARC outlet increase from ‌5.33 % to 9.28 % and from 14.56 % to 21.91 %, respectively. An optimal equivalence ratio exists for maximizing H₂ concentration at ARC outlet. NO_x concentration decreases from 71 ppm to 6 ppm when the equivalence ratio increases from ‌1.30 to 1.83.