In order to discover reaction mechanism between ammonia and NOx, reaction process of ammonia and NOx was simulated from the point of kinetics. As a result, NOx removal efficiency was kept in 47.23% ~98.89% at 800°C~1000°C. When NH3/NO was equal or less than 1.5, NOx removal efficiency was enhanced obviously with NH3/NO increasing. NH2 was produced as an important intermediate product in NH3 decomposition. Firstly NH2 was formed in reactions between NH3 and free radicals such as OH, H, O, M. Then NO was directly reduced to N2 by NH2.Therefore, free radicals (especially for NH2, O and H) have important influence on removal efficiency of NOx during ammonia injection.