A laboratory-scale anaerobic-anoxic-aerobic process (A O) with a small aerobic zone and a bigger anoxic zone and biologic aerated filter (A O-BAF) system was operated to treat low carbon-to-nitrogen ratio domestic wastewater. The A O process was employed mainly for organic matter and phosphorus removal, and for denitrification. The BAF was only used for nitrification which coupled with a settling tank Compared with a conventional A O process, the suspended activated sludge in this A O-BAF process contained small quantities of nitrifier, but nitrification overwhelmingly conducted in BAF. So the system successfully avoided the contradiction in sludge retention time (SRT) between nitrifying bacteria and phosphorus accumulating organisms (PAOs). Denitrifying phosphorus accumulating organisms (DPAOs) played an important role in removing up to 91% of phosphorus along with nitrogen, which indicated that the suspended activated sludge process presented a good denitrifying phosphorus removal performance. The average removal efficiency of chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), and were 85.56%, 92.07%, 81.24% and 98.7% respectively. The effluent quality consistently satisfied the national first level A effluent discharge standard of China. The average sludge volume index (SVI) was 85.4 mL·g additionally, the volume ratio of anaerobic, anoxic and aerobic zone in A O process was also investigated, and the results demonstrated that the optimum value was 1∶6∶2.