Abstract: Taking municipal waste water as treatment object,a step feeding-multistage A/O process for nitrogen removal was conducted.Furthermore,the aim of this paper was to research growth characteristic of Tubificidae,and its affection to nitrogen removal and sludge charactenstics.After 120 days operation,a good performance was achieved in the system.The COD、NH₄⁺-N and TN removal efficiency was 84%、100% and 70%-80%,respectively.Three above indexes could meet Standard A of Level 1 in "Discharge Standard of Pollutants for Municipal Wastewater"(GB18918—2002).It was found that low DO concentration was unfit for tubificidae growth,and the mature Tubificidae required more facultative space than the larvaes in the long-time operating process.Tubificidae growth didn't affect the removal ability of the system.And,it could increase the sludge sedimentation performance and promoted sludge reduction in a certain degree.