Complete nitritation process in an biofilm moving bed system was started-up by inoculating nitrobacteria and controlling pH, and the effects of nitrogen loading rate (NLR) and hydraulic retention time (HRT) on the stability of the system were investigated. The results showed that the system could achieve complete nitritation after 10 day's acclimation by controlling pH within the range of 7.7 - 8.2, under the conditions of temperature (30 +/- 1) degrees C, DO 1.5 - 2.0 mg/L, HRT 24 h,ammonia concentration 150 mg/L. Conversion rate of ammonia was above 96% and nitritation rate (NO2(-) -N/NO(x)(-) -N) was more than 95%. In addition, the system remained complete nitritation when NLR increased from 0.15 kg/(m3 x d) to 0.24 kg/(m3 x d), and conversion rate of ammonia was still above 90%, nitritation rate maintained at 96%, but the type of nitrification turned from completely nitritation to completely nitrification under the condition of over aeration by extended HRT under low NLR. However, it could be resumed by shortened HRT.