Abstract: Complete ammonia oxidation (comammox) is the process of directly oxidizing ammonia to nitrate by a single microorganism. The discovery of comammox breaks the conventional view that the nitrification process is divided into two steps, suggesting that the nitrification process can be catalyzed by a single microorganism. Comammox bacteria might have important effects on the research of microbial nitrogen metabolism, earth nitrogen cycle and sewage nitrogen removal. The discovery process of comammox bacteria was outlined, and the physiological and ecological characteristics and environmental distribution characteristics of comammox bacteria were summarized in this paper. Moreover, the existence of comammox microorganisms in sewage biological treatment systems and their potential impact on nitrogen transformation were discussed. In particular, comammox bacteria had a competitive advantage under low oxygen and ammonia conditions, suggesting its potential impact on the stability of partial nitrification and anaerobic ammonia oxidation process. Finally, the future research of comammox bacteria in sewage biological treatment system was proposed.