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.