Abstract: The Naididae are common oligochaete worms in activated sludge. In this study, individuals of Naididae were inoculated in activated sludge under suitable conditions, and their reproduction mode and reproduction rate, as well as the structure of activated sludge flora before and after feeding were investigated. The results show that germination reproduction dominates in the asexual reproduction of Naididae in activated sludg, forming mother and daughter after division. The average continuous lysis time of the mother is 37.3 h, the average time of the daugher’s first lysis is 49.2 h, and its average doubling time is 21.2 h at 25 ℃. Besides, the feeding behavior of the Naididae leads to the decrease of denitrifying and phosphorus removing flora in activated sludge, and the relative increase of organic matter degrading flora. Consequently, its massive proliferation will reduce the removal effect of nitrogen and phosphorus in the activated sludge system.