To study the effect of alkalinity on partial nitrification(PN) and the feasibility to control the ratio of nitrite to ammonia in effluent, the PN was rapidly started-up in a sequencing batch reactor. The ammonia conversion rate, ammonia oxidation rate and the microbial activity were investigated in different ratio of the inflow alkalinity to ammonia. Results showed that the PN could be achieved quickly by pre-cultivating the sludge with high ammonia nitrogen, and over 96％ nitrite accumulation rate was sustained. The rate can be controlled flexibly with the liner relationship between ammonia conversion and the ratio of alkalinity to ammonia while alkalinity is insufficient. Cycling experiments show a good indication of alkalinity on nitrite, the alkalinity less than 50 mg/L will stop the ammonia oxidation, and the inorganic carbon less than 3.0 mmol·g-1 will lead to the reduction of microbial population and activity. In actual operation, the ratio of nitrite to ammonia in effluent can be effectively controlled by the alkalinity.