The nitrite accumulation in Biological Aerated Filter (BAF) treating simulated municipal wastewater was investigated and the possibility of nitrous nitrification in low ammonia wastewater treatment was discussed. The results indicate that the lower concentration of DO was of advantage to the formation of the nitrite accumulation. When the concentration of DO was controlled to less than 0.5 mg/L, the nitrite accumulation rate can improved significantly to the maximum over 80%. By adjusting pH, nitrite accumulation can take place in BAF. When the influent pH value was 6.5 to 7, the highest nitrite accumulation rate was 57%. But long-term stable nitrite accumulation cannot be obtained only through changing influent pH value. The increasing of hydraulic loading can contribute to nitrite accumulation. When the hydraulic loading was enhanced to 10 m3/(m2•h) , the average nitrite accumulation rate can be increased by 52.5 times. When concentration of DO was 1.0 to 1.5, influent pH was 6.5 to 7 and the filtration rate was 5 m/h, the nitrite accumulation of BAF reactor was of good stability. During 60 days’ running, the nitrite accumulation rate can keep at 65% to 75% stably and obvious nitrous nitrification was found.