Through the test on activated sludge in SBR reactor,the influence of DO on nitrification is studied.First,through a 195d continuous test,to maintain the DO concentration of nitrifying system at a low level,if the DO is in range between 0.2 mg/l(start) ~0.6 mg/l(end) there will be high NH+4-N reduction and the nitrification ratio will reach 50~90%;Second,according to the regularity of DO in a cycle,12h as a react time is proper.If the react time of 16h is taken,it will be a traditional nitrification.At last,through bottle reactors,it can be concluded that when the DO concentration of system rise,NH+4-N the reduction ratio will rise too,but the nitrification ratio will drop,the NH+4-N reduces and the nitrification ratio can be calculated from a formula.