Abstract: Two sequencing batch reactors(SBR) treating domestic sewage with low COD / TN ratio were employed to investigate the realization and characterization of alternate oxic-anoxic nitritation. The results show that stable nitritation is realized on the condition of temperature of(24 ± 2) ℃,sludge retention time of 35 day and without aeration restriction. And the nitrite accumulation rate of effluent is above 90%,with no ammonia and little nitrate(lower than 2 mg / L) in effluent. The specific ammonia oxidation rates of the SBRs being operated under alternate mode for 200 days are respectively twice and1. 8 times of those under oxic-anoxic mode,thus aeration duration is shortened and energy consumption of aeration is saved without impacting effluent quality. The performance of the SBRs operated under alternate mode is improved,and the ammonia,COD and TN removal efficiency reach 100%,80% and70% respectively.