Abstract: To further investigate the effect of dissolved oxygen (DO) on microbial community of nitrify bacteria in the activated sludge, the microbial community of enriched nitrify sludge under the different DO level was analyzed by high-throughput sequencing technology. Results show that for nitrify sludge including both ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB), low-DO condition increase the microbial diversity, and high-DO enriches the nitrifying functional bacteria ( Nitrosomonas, Nitrospira). For nitrify sludge only including NOB, there is no obvious difference of microbial diversity between low-DO condition and high-DO condition, and Nitrospira prefers to grow under the low-DO condition. Moreover, different DO conditions also affect the microbial community of other bacteria except nitrifyer.