Anaerobic/anoxia sequencing batch reactor (A/ASBR) system was used to analyze the quantity and composition of each branch of phosphorus accumulating organisms (PAOs) in activated sludge under different nitratenitrogen (NO3 -N) concentrations by using real-time quantitative polymerase chain reaction (PCR) technology. The study determined whether NO3 -N and its concentration change were the main driving factors for the variation of the quantity and composition of each branch of PAOs. The results show that with the increase of NO3 -N concentration from 10mg/L to 40mg/L, the number of bacterial 16S rRNA genes in the A/ASBR reactor changed slightly at 6.81× 1011-7.53×1011 copies/g dry sludge. The number of PAO genes (Acc 16S rRNA) increased from 1.98×1011 to 3.53×1011 copies/g dry sludge, and the total number of ppk1 genes increased from 1.25×1011 to 3.59×1011 copies/g dry sludge. Additionally, the number of polyphosphate kinase (ppk) genes in Accumulibacter branch IA, IIC and IID was high, and the changes were positively related to the concentration of NO3 -N, while the number of branches in IIA, IIB and IIF was very low. The dosing concentration of NO3 -N was the main driving factor for the change of PAOs and their branch number and composition in the A/ASBR reactor.