Stable short-cut nitrification was achieved through the selective inhibition of free ammonia (FA), free nitrous acid (FNA) on the nitrifying bacteria and a real-time control strategy which used pH, DO and ORP as parameters in an SBR reactor. The effects of C/N ratio and initial pH value on the short-cut biological nitrogen removal were studied. It is found that: when influent NH4 +-N concentration was 108~177.3 mg/L, the nitrite accumulation rate was around 90%. The reasons for the favorable nitrite accumulation rate were both the inhibition of FA and FNA on the NOB and the real-time control strategy through detecting the “ammonia valley” and “nitrite knee” two feature points in pH and ORP profiles, respectively. The optimal C/N ratio was 3 when acetate was used as the carbon source. When the denitrification rate with respect to the concentration of mixed liquor suspended solids was 19.8 mg·g -1·h -1 NOx --N,NH4 +-N, NO2 --N, NO3 --N, TN of the effluent were less than 6, 2, 1 and 30 mg/L, respectively. When initial pH value was 8.5, the denitrification rate was maximum. However the differences of denitrification rate were small when pH was in the range of 7.5~8.5.