A　growing　interest　has　emerged　in　carbon　dot　(CDs)　fluorescent　probes,　as　they　are　non-destructive,　non-contact,　sensitive,　fast,　and　low-cost.　However,　the　low　fluorescence　quantum　yield　(QY)　and　the　uncertainty　of　fluo-rescence　quenching　mechanism　are　the　main　bottlenecks　restricting　its　application.　Here,　we　used　mustard　seeds　as　carbon　source,　CDs　were　synthesized　by　a　simple　hydrothermal　method　without　further　surface　treatment　with　the　QY　reached　a　value　as　high　as　17.92　%　in　468　nm　excitation　energy.　As　a　label-free　fluorescent　probe,　CDs　can　be　used　to　sense　Fe3+　ions　in　environmental　water　and　human　oral　keratinocytes　(Hok)　cells,　with　a　detection　limit　of　0.51　mu　M　and　the　linear　range　of　0-75　mu　M.　Moreover,　we　have　conducted　an　in-depth　study　on　the　mechanism　of　fluorescence　quenching　of　CDs　caused　by　Fe3+　ions.　Our　study　provides　powerful　evidences　that　the　mechanism　of　fluorescence　quenching　to　Fe3+　ions　is　a　synergistic　effect　of　static　quenching　(SQ)　and　aggregation-induced　effect　(AIE),　which　accompanied　by　forming　a　non-fluorescent　ground　state　complex　be-tween　CDs　and　Fe3+　ions.　This　work　promises　to　provide　new　ideas　for　the　most　appropriate　detection　strategy　and　design　the　best　performing　sensor,　allowing　their　effective　application　in　the　sensing　field.