Partial nitrification (PN) process was successfully developed in a plug flow reactor fed with low strength sewage at ambient temperature (18-21.5℃). For better detecting the efficiency and mechanism of the partial nitrification process in the PN reactor, ammonia, nitrite, and nitrate were detected firstly, subsequentially the functional bacterial community in the reactor was also investigated at microbial level. The sludge microstructures were detected by scanning electron microscopy (SEM), and the microbial characteristics was studied via fluorescence in situ hybridization (FISH), denaturing gradient gel electrophoresis (DGGE), cloning and sequencing analyzing. The DO concentration was maintained between 0.1 to 0.6 mg/L, to enhance AOB competition superiority. After 80 days continuous operation, PN reactor achieved nitrite accumulation rate of 100% and effluent NO₂^--N to NH₄⁺ -N ratio achieved 1.11. SEM results showed that spherical bacteria were the predominant bacteria in partial nitrifying sludge. FISH results showed the proportion of AOB and NOB were 37.3% and 4.4%, respectively. Bacterial DGGE and sequencing results indicated that six dominant bacteria coexisting in the PN reactor, among which Nitrosomonas sp. were the main AOB species.Those multiple functional bacteria contributed to the nitrogen removal in PN reactor.