Abstract: In order to effectively control the sludge filamentous bulking in A/O phosphorus removal process when treating wastewater of septic tank.By changing sludge loading、dissolved oxygen concentration,and reducing the concentration of S^2-,sludge filamentous bulking was controlled effectively,and also the settling ability of activated sludge got recovered.The results showed that,when the sludge loading of A/O process is higher,serious sludge filamentous bulking easily happened,because the remaining COD was still very high when it got out of anaerobic tank,which led to disadvantaged PAO when it competed with filamentous bacteria.When A/O process ran at lower sludge loading,the problem of slight sludge filamentous bulking still existed,because the amount of sulfide in the wastewater was very high,which triggered the proliferation of filamentous sulfur bacteria.