Abstract: To study the effects of influent ρ(C)/ρ(N) on two-sludge nitrogen and phosphorus removal system, the characteristics of a pilot-scale AAO-biological aerated filter(BAF) treating about 50 m³ real domestic wastewater per day were investigated under ρ(C)/ρ(N) level of 3.3±0.3, 4.5±0.3, and 6.0±0.3, respectively. Results show that when ρ(C)/ρ(N) is about 3.3, the removal rates of COD, TN and PO₄^3--P reach 71.4%, 67.6% and 85.6%, respectively. The nitrogen and phosphorus removal performance of the system reach the best when ρ(C)/ρ(N) is raised to 4.5 by adding organic matter, the removal rates of COD, TN and PO₄^3--P reaching 79.7%, 70% and 93.5%, respectively. The average effluent TN and PO₄^3--P are 12.40, 0.20 mg/L, respectively. However, when ρ(C)/ρ(N) is about 6.0, a large amount of available organic compounds in the anoxic area result in the rapid depletion of the electronic receptor NO₃^--N by denitrifying bacteria, affecting the removal of the nitrogen and phosphorus by DPAOs. As well, residual organic matter is found in the BAF, which result that NH₄⁺-N cannot be completely oxidized to NO₃^--N because of the proliferation of heterotrophic bacteria. In this situation, the removal rate of COD is 81.6%, removal rates of TN and PO₄^3--P by AAO-BAF process drop to 55.1% and 63.2% rapidly, and the system is close to collapse.