The effects of dissolved oxygen(DO),mixed liquid suspended solids(MLSS) and sludge return ratio(R) on nitrogen and phosphorus removal of improved oxidation ditch were studied through orthogonal experimental.The results of range analysis show that the importance sequences of the factors influencing the removal of CODCr,TN,TP are DOMLSS R,DORMLSS,MLSSRDO,respectively.The results of variance analysis show that,DO and MLSS have significant influence on decarburization,and DO also has significant influence on nitrogen removal,MLSS and R have significant influence on phosphorus removal.In the end,it is obtained that,the best operating conditions of improved oxidation ditch for nitrogen and phosphorus removal is that the concentration of anoxic zone and aerobic zone of improved oxidation ditch was 0.3 to 0.5mg/L and 2.0~2.5mg/L,MLSS is 5000mg/L,sludge return ratio R is 65%.