In　order　to　study　the　relationship　of　microorganisms　in　bioreactor　and　enhance　nitrogen　removal,　influence　of　pH,　dissolved　oxygen　and　applied　loading　on　biochemical　reaction　and　population　optimizing　were　studied.　The　experimental　results　show　that　controlling　loading　could　make　two　stage　filters　operated　at　the　optimized　conditions　respectively　due　to　population　competition.　Maintaining　long　time　shortcut　nitrification　is　not　likely　to　be　feasible　when　the　pH　value　is　below　8.5.　Nevertheless,　utilizing　the　characteristic　that　specific　growth　rate　of　ammonia　oxidizer　is　higher　than　nitrite　oxidizer　at　low　dissolved　oxygen　concentrations,　the　nitrite　accumulation　could　be　up　to　84%　at　the　certain　loading　if　the　concentration　of　dissolved　oxygen　in　the　system　is　controlled　below　1.8　mg/L.　It　is　beneficial　to　the　design　of　the　shortcut　nitrification-denitrification　reactors.