Abstract: A Membrane Bioreactor (MBR) has widely been used as the biological treatment with membrane filtration in municipal and industrial wastewater, due mainly to the better water quality of product and long Sludge Retention Time (SRT). In this study, a pilot-scale partial nitrosation process was carried out in a membrane bioreactor to achieve a stable accumulation of nitrite nitrogen. Scientific operating parameters were obtained to realize that the nitrosated and anaerobic digestion quickly started and ran stably in a pig farm. The results showed that the Dissolved Oxygen (DO) level was controlled in the reactor at 0.2-0.5 mg/L under the normal temperature, and the pH was controlled at 8.0±2.0. The partial nitrosation process was successfully started and operated stably, where the accumulation rate of nitrite nitrogen (NAR) reached the maximum of 87.95% and remained stable at a high level. The concentrations of nitrite nitrogen and ammonia nitrogen in the effluent were (197.68±27.51) mg/L and (215.61±33.91) mg/L, respectively. In the stable operation stage, the concentration of NO3--N in the effluent gradually decreased and stabilized at about 30 mg/L. By adjusting the intermittent aeration ratio in a reactor to 20 min: 20 min, the degree of ammonia oxidation was controlled, while about 50% of NH4+-N in the influent water was oxidized to NO2--N by Ammonia Oxidizing Bacteria (AOB). A strong suggestion was made to inhibit the Nitrite Oxidizing Bacteria (NOB) from oxidizing NO2--N to NO3--N, thereby maintaining the accumulation rate of NO2—N and stabilizing the ρ(NO2--N)/ρ(NH4+-N) of the effluent at 1.1:1, which were well suitable for the substrate requirements for anammox reaction. During the partial nitrosation process, the AOB activity improved significantly and maintained at 0.4±0.02 g/(g•d), the NOB activity increased slightly and then decreased to remain at a low level. The AOB activity was significantly higher than the NOB activity. The microbial diversity increased in the reactor, and the flora structure changed significantly. The dominant bacteria in the reactor shifted from Chloroflexi and Firmicutes to Proteobacteria. At the genus level, Nitrospira did not increase significantly, while the relative abundance of Nitrolancea gradually decreased during the entire operation, and the relative abundance of Nitrosomonas showed an upward trend. Nitrosomonas became the dominant genus in the reactor after partial nitrosation was successfully started, the relative abundances of Zobellella, Subdivision5_genera_incertae_sedis, and Smithella increased slightly, indicating that there were other denitrification reactions in the reactor that acted on the removal of ammonia nitrogen. The relative abundance of NOB in the reactor was significantly higher than that of AOB at the initial stage of a startup. Increasing the concentration of Free Ammonia (FA) promoted the increase of the relative abundance of AOB and NOB, and the ratio of AOB/NOB increased from 0.26 to 1.68. After a stable operation, the ratio of AOB/NOB in the nitrosation tank was 5.07. The AOB showed the main advantage in the partial nitrosation after the start-up. The increase in the ratio of AOB/NOB promoted the accumulation of NO2--N.NAR, water temperature, and FA, where the NO2--Neff/NH4+-Neff were the main factors that changed the distribution of microbial communities. The lower FA and temperature were not conducive to the accumulation of nitrite nitrogen, whereas, the higher FA concentration and temperature promoted the accumulation of nitrite. The finding can provide technical support for the application of part nitrosation and anammox process in actual engineering.