A pilot-scale, pre-anoxic-anaerobic oxidation ditch was used in this study to treat municipal wastewater with limited carbon source. A novel return activated sludge (RAS) pre-concentration tank was adopted for improving the phosphorus removal efficiency and the effects of RAS pre-concentration ratio were studied. Under the optimal operational condition, the suspended total phosphorus (STP) and the total phosphorus (TP) removal efficiencies were around 58.9% and 63.9% respectively and the effluent PO43- -P was lower than 0.8 mg-L-1. The reason is that with the optimal RAS pre-concentration ratio, nitrate is completely removed with endogenous carbon source and the secondary phosphorus release is strictly restrained in the pre-anoxic tank. Therefore, the anaerobic phosphorus release and the carbon source uptake by phosphorus accumulation organisms (PAOs) in the sludge, which are extremely important to the phosphorus removal process, can be fully satisfied. Furthermore, the oxidation-reduction potential is proved to be suitable for controlling the RAS pre-concentration ratio due to influent fluctuation and varied conditions. The novel modified system is also beneficial for PAO accumulation.