The　structure　and　function　of　activated　sludge　community　were　studied　in　an　A/O　pilot-scale　plant　treating　actual　domestic　wastewater.　FISH　results　show　the　feasibility　and　effectiveness　of　sludge　population　optimization.　A/O　process　short-cut　nitrification　with　process　control　of　DO　and　aeration　based　on　DO　and　pH　on-line　sensors　can　be　achieved,　and　gradually　eliminated　nitrite　oxidising　bacteria,　and　therefore　achieved　nitrogen　removal　via　the　nitrite　pathway.　It　can　be　realized　sludge　population　optimization,　improving　nitrogen　removal　and　saving　operational　cost　at　the　best　with　on-line　process　control.