Profiles　of　oxidation　reduction　potential　(ORP)　and　pH　value　were　investigated　during　organic　degradation,　nitrification　and　denitrification　of　advanced　nitrogen　removal　in　step-feed　cyclic　activated　sludge　technology　(CAST)　with　on-line　control　treating　domestic　wastewater,　and　the　relationship　among　process　parameters　and　variations　of　organic　substances　and　nitrogens　during　the　treatment　process　was　also　established.　The　results　show　that　based　on　the　characteristic　points　in　pH　and　ORP　profiles　to　halt　aeration　and　stirring　process　timely,　step-feed　CAST　with　on-line　control　system　is　found　to　perform　more　effectively　so　that　advanced　nitrogen　removal　is　achieved　and　operation　cost　is　saved　as　much　as　possible.　When　COD　and　mass　concentrations　of　-N　in　the　influent　are　155.0-443.6　mg/L　and　57.98-82.40　mg/L,　COD　and　mass　concentrations　of　-N　and　TN　in　the　effluent　are　less　than　40,　0.5　and　2.0　mg/L,　respectively,　as　step-feed　CAST　with　on-line　control　adopted.　Moreover,　the　specific　denitrification　rate　is　observed　to　be　improved　significantly　when　the　temperature　of　system　is　17　°C,　23　and　30　°C.　Sufficient　organic　carbon　in　influent　can　increase　feeding　frequencies,　shorten　treatment　process　as　well　as　decrease　external　carbon　source　dosage.　The　phosphorus　removal　performance　is　stable　and　the　removal　rate　can　reach　90%　when　step-feed　CAST　technology　is　used.