Owning　to　the　large　amounts　of　factors　for　causing　filamentous　bulking　sludge　and　many　troubles　in　expressing　its　mechanistic　model　of　causing　filamentous　sludge　bulking,　a　simplified　mechanistic　model　is　proposed　for　sludge　volume　index(SVI)　from　the　filamentous　bacteria　growth　kinetics　in　this　paper.　First,　the　appearance　of　filamentous　sludge　bulking　is　studied.　And　then　the　relationships　among　the　SVI　and　the　impact　factors　are　obtained　by　analyzing　the　filamentous　sludge　bulking　process.　Secondly,　the　main　factors　for　SVI　are　ensured.　The　simplified　mechanistic　model　of　SVI　is　designed.　Meanwhile,　the　parameters　of　this　proposed　model　are　adjusted　by　using　the　method　of　data　analysis　and　statistics.　Finally,　this　simplified　mechanism　model　is　used　for　a　real　wastewater　treatment　process.　The　results　show　that　this　proposed　model　can　simulate　the　mechanistic　of　filamentous　sludge　bulking　and　predict　SVI　accurately.　©　All　Rights　Reserved.