This study used short process nitrification sludge to investigate the effect of high free ammonia (FA) on ammonia escape during ammonia oxidation. A sequencing batch reactor (SBR) was used to study the ammonia escape process under different FA concentrations. The results indicate that FA in water is usually combined with water molecules at lower FA concentrations to produce stable NH3·H2O with almost no ammonia escape. With high FA concentration (FA>687.1 mg·L-1), ammonia nitrogen is not oxidized to oxidized nitrogen (NO2--N concentration is <0.1 mg·L-1 at the end of aeration), but the total nitrogen (TN) reached 269.7 mg·L-1. Therefore, when there is a high FA concentration, NH4+-N will evaporate in the form of FA, resulting in ammonia escape. In addition, when 226.6 ≤ FA ≤ 711.8 mg·L-1, the free ammonia escape rate (FEV) grows rapidly with the increase in FA concentration.