The　overgrowth　of　filamentous　fungi　leading　to　sludge　bulking　constitutes　a　significant　threat　to　the　structural　and　functional　stability　of　aerobic　granular　sludge　(AGS).　This　study　investigated　the　inhibitory　effects　of　magnetic　powder　(Fe3O4)　on　filamentous　fungi,　analyzed　its　toxic　mechanism,　and　evaluated　its　effectiveness　in　mitigating　sludge　bulking.　Results　indicated　that　the　inhibitory　effects　of　Fe3O4　were　closely　related　to　its　concentration　and　particle　size.　Different　concentrations　and　particle　sizes　of　Fe3O4　led　to　a　regular　decrease　in　both　fungi　and　bacteria　populations,　with　a　significant　reduction　in　fungi　population.　The　relative　abundance　of　filamentous　fungi　decreased　from　98.01　%　(0　g/L　Fe3O4)　to　0.32　%-7.19　%　(1.0　g/L　Fe3O4),　indicating　the　toxicity　of　Fe3O4　in　both　fungi　and　bacteria.　The　toxic　mechanism　was　attributed　to　the　effects　of　Fe3O4　on　the　microbial　interface.　The　change　in　sludge　surface　properties　promoted　the　physical　penetration　of　Fe3O4,　resulting　in　oxidative　stress　and　membrane　damage　of　microorganisms.　Filamentous　fungi　were　more　susceptible　to　toxicity　than　bacteria　due　to　their　larger　specific　surface　area　and　extracellular　polymeric　substance　coating,　which　facilitated　adhesive　contact　with　Fe3O4,　providing　more　points　for　penetration.　This　study　explored　how　Fe3O4　inhibits　filamentous　fungi　in　the　mixed　fungi-bacteria　systems,　proposed　a　conceptual　model　for　the　action　of　Fe3O4　on　filamentous　fungi　surface,　and　established　an　effective　method　to　restore　sludge　bulking,　which　provided　a　solution　to　the　AGS　disaggregated　problem　caused　by　filamentous　fungal　overgrowth.