Abstract: In this study, Modified University of Cape Town (MUCT) process was used to treat domestic sewage with low ρ(C)/ρ(N) ratio, and the relationship between population dynamics of dominant Candidatus Accumulibacter and efficiency of denitrifying phosphorus removal under nitration and nitrification was studied. Flow cytometry based quantification showed that Accumulibacter accounting for 22.1% of total bacteria were dominant phosphate accumulating organisms (PAOs) in system under nitrification. qPCR based estimation indicated that dominant species IIC, IIC(ppk1 excluding OTU NS D3) and IIF grew fast and accounted for 11.53%, 34.31% and 17.97% of Accumulibacter, respectively, while their ability using nitrite to achieve denitrifying phosphorus removal were stronger than that of other clades and were important for nitrite removal in system. As the most important clade in N and P removal in system, IID was always dominant along entire operation, which accounted for 80% of Accumulibacter under nitrification, but not as competitive as other Type Ⅱ clades under nitration.