Based on a laboratory-scale anaerobic-anoxic-oxic process acclimated with municipal wastewater, effect of SRT on denitrifying phosphorus removal occurred in the system was investigated. The results indicate that the contribution of anoxic denitrifying phosphorus removal to total phosphorus removal becomes larger with prolonging the sludge retention time. DNPAOs' denitrification and P uptake capacities per anoxic PHAs utilized are quickly increased, while PAOs' P uptake capacity per aerobic PHAs utilized is rarely influenced by SRT. When SRT is set at 12 days, anoxic phosphorus uptake efficiency and biological nutrient removal efficiency are best. The results also show that the COD amountrequired for removing per nitrogen decreases and for removing per phosphorus increases with prolonging the sludge retention time. For the national typical municipal wastewater, the COD amount required for removing per nitrogen and phosphorus under SRT of 12 and 15 days is less than that of under SRT of 8 days, which indicates that anoxic denitrifying phosphorus removal can really decrease the demand of carbon and energy sources.