In　order　to　effectively　control　the　sludge　filamentous　bulking　in　A/O　phosphorus　removal　process　when　treating　wastewater　of　septic　tank.　By　changing　sludge　loading,　dissolved　oxygen　concentration,　and　reducing　the　concentration　of　S2-,　sludge　filamentous　bulking　was　controlled　effectively,　and　also　the　settling　ability　of　activated　sludge　got　recovered.　The　results　showed　that,　when　the　sludge　loading　of　A/O　process　is　higher,　serious　sludge　filamentous　bulking　easily　happened,　because　the　remaining　COD　was　still　very　high　when　it　got　out　of　anaerobic　tank,　which　led　to　disadvantaged　PAO　when　it　competed　with　filamentous　bacteria.　When　A/O　process　ran　at　lower　sludge　loading,　the　problem　of　slight　sludge　filamentous　bulking　still　existed,　because　the　amount　of　sulfide　in　the　wastewater　was　very　high,　which　triggered　the　proliferation　of　filamentous　sulfur　bacteria.