Modeling for nitritation proce ss wa s discussed and analyzed quantitatively for the factors that influence nitrite accumulation. The re sults indicated that pH, inorganic carbon source and Hydraulic Retention Time (HRT) a s well a s bioma ss concentration are the main factors that influenced the conversion ratio of ammonium to nitrite. A constant high pH can lead to a high nitritation rate and re sults in high conversion ratio on condition that free ammonia inhibition do not happen. In a CSTR system, without pH control, this conversion ratio can be monitored by pH variation in the reactor. The pH goe s down far from the inlet level means a strongly nitrite accumulation. High concentration of alkalinity can promoted the conversion ratio by means of accelerating the nitritation rate through providing sufficient inorganic carbon source (carbon dioxide). When inorganic carbon source wa s depleted, the nitritation proce ss stopped. HRT adjustment could be an efficient way to make the nitritation system run more flexible, which to some extent can meet the requirements of the fluctuant of inlet parameters such a s ammonium concentration, pH, and temperature and so on. Bioma ss concentration is the key point, e specially for a CSTR system in steady state, which wa s normally circumscribed by the characteristics of bacteria and may also affected by aeration mode and can be increa sed by prolonging the HRT on the condition of no nitrate accumulation when no recirculation available. The higher the bioma ss concentration is, the better the nitrite accumulation can be obtained.