Given　the　significant　impact　of　Fe3+　and　ascorbic　acid　(AA)　on　human　health,　the　development　of　an　efficient　sensor　for　their　detection　is　of　critical　importance.　In　this　study,　a　novel　fluorescent　probe　composed　of　ultrathin　boron　nanosheets　(Ultrathin　BNSs)　is　introduced　for　the　sensitive　detection　of　both　Fe3+　and　AA　in　human　serum.　Ultrathin　BNSs,　exhibiting　a　fluorescence　quantum　yield　of　up　to　6.8　%　and　minimal　cytotoxicity,　can　be　directly　synthesized　from　boron　through　an　ultrasound-assisted　liquid-phase　exfoliation　(LPE)　method.　These　Ultrathin　BNSs　demonstrate　a　selective　and　responsive　fluorescence　＂on-off-on＂　mechanism　toward　Fe3+　and　AA,　governed　by　the　induction　and　removal　of　an　electron-transfer　effect.　The　presence　of　Fe3+　induces　non-radiative　electron　transfer,　resulting　in　fluorescence　quenching　of　the　Ultrathin　BNSs.　In　contrast,　the　introduction　of　AA　facilitates　the　dissociation　of　Fe3+　from　the　Ultrathin　BNSs/Fe3+　complex,　leading　to　fluorescence　recovery.　When　utilized　as　a　fluorescent　nanoprobe,　Ultrathin　BNSs　exhibit　a　strong　linear　correlation　with　Fe3+　concentrations　ranging　from　0.2　to　150　mu　M　and　AA　concentrations　between　1.5　and　110　mu　M.　Additionally,　the　limits　of　detection　(LOD)　for　Fe3+　and　AA　reach　0.2　mu　M　and　1.5　mu　M,　respectively,　demonstrating　superior　performance　compared　to　numerous　previously　reported　fluorescent　probes.　The　feasibility　of　this　Ultrathin　BNSs-based　sensor　for　AA　detection　in　human　serum　was　further　examined,　yielding　satisfactory　outcomes.　It　is　expected　that　our　strategy　may　offer　a　new　approach　for　developing　rapid,　low　cost　and　sensitive　ultrathin　BNSs-based　sensors　for　biological　sensing　and　environmetal　applications.