Abstract: Taking synthetic wastewater of the high ammonia nitrogen and low C/N ratio as the influent, the membrane bio-reactor (MBR) was used to study establishment of stable nitritation process. The main parameters of the reactor were controlled as follows: the temperature of nitritation tanks was 28 - 30 ℃, dissolved oxygen (DO) was 0.5 mg/L, HRT was 12 h, pH was 7.8 - 8.0, influent ammonia concentration was 200 mg/L, COD_Cr was 40 mg/L. Under these conditions, the nitritation of wastewater with C/N of 1:5 was realized in the nitritation tanks. After 14 days, sludge age was 100 days, a stable nitrite nitrogen accumulation was achieved. When the ammonia concentration was increased to 400 and 800 mg/L, respectively, the tolerance of nitrosobacteria against shock of high ammonia load was relatively high.