The influence of dissolved oxygen(DO), C/N and pH on simultaneous nitrification and denitrification in the membrane bioreactor was studied. The experimental results showed that only under condition that each ecological factor was maintained relative steady, simultaneous nitrification and denitrification might proceed smoothly. When DO, C/N and pH of feed water was 1 mg/L, 30 and 7.2, the removal rate of COD, NH4+, and TN were 96%, 95% and 92% respectively.