This study investigates the effect of increased hydraulic retention time (HRT) and dissolved oxygen (DO) on the destruction and restoration of the performance of activated sludge and biofilm sequencing batch reactor (SBR) nitritation after the processes were simultaneously started up and stably operated at room temperature (25℃). The results showed that was easier to start nitritation in an activated sludge reactor, but nitritation was destroyed when DO was 2-2.5 mg·L-1. The performance could be restored by reducing DO to 0.5-1 mg·L-1. The biofilm process was not affected by DO. Prolonging HRT destroyed the stable operation of the two processes. Compared with the method of biofilm, the activated sludge method had less resistance, but restoring performance was better than for the biofilm process after shortening the HRT. Subsequently, the temperature (20, 15, and 10℃) was continuously reduced to investigate the synergistic effect of DO and temperature on nitritation stabilization. The results showed that the reduction of temperature destroyed the stable operation of nitritation, but the reduction of DO concentration could compensate for the adverse effects of temperature reduction. In addition, when the temperature was higher than 20℃, the rate of nitritation in the activated sludge process was better than that in thebiofilm method. At low temperature (below 15℃), a stable operation of nitritation was easier to achieve with the biofilm method. The capacity of the activated sludge process was hardly changed at 10℃. The above conclusions were confirmed by molecular microbiological analysis, and it was found that the stable operation of nitritation could be achieved without complete wash-out of the nitrite-oxidizing bacteria (NOB).