A　novel　displacement-type　quartz　crystal　microbalance　(QCM)　immunosensing　strategy,　based　on　glucose　and　its　analogue　dextran　for　concanavalin　A　(ConA)　binding　sites,　was　designed　for　ultrasensitive　monitoring　of　small　molecular　biotoxins　(brevetoxin　B,　PbTx-2,　used　as　a　model)　with　signal　amplification　on　a　graphene-functionalized　sensing　interface.　To　construct　such　a　QCM　immunosensing　platform,　phenoxy-functionalized　dextran　(DexP)　was　initially　assembled　onto　the　surface　of　graphene-coated　QCM　probe　via　the　pi-stacking　interaction,　and　ConA-labeled　monoclonal　mouse　anti-PbTx-2　capture　antibody　was　then　immobilized　on　the　DexP-modified　probe　by　dextran-ConA　binding.　Gold　nanoparticle　heavily　functionalized　with　glucoamylase　and　bovine　serum　albumin-PbTx-2　(PbTx-2-BSA)　conjugate　was　employed　as　the　trace　tag.　A　competitive-type　immunoassay　format　was　adopted　for　the　online　monitoring　of　PbTx-2　between　anti-PbTx-2　antibody　immobilized　on　the　QCM　probe　and　PbTx-2-BSA　labeled　on　the　gold　nanoparticle.　Accompanying　the　gold　nanoparticle,　the　carried　glucoamylase　could　hydrolyze　amylopectin　in　glucose.　The　produced　glucose　competed　with　dextran　for　ConA　and　displaced　the　ConA-streptavidin-anti-PbTx-2　complex　from　the　QCM　probe,　resulting　in　the　frequency　change.　Under　optimal　conditions,　the　frequency　of　the　QCM　immunosensor　was　indirectly　proportional　to　the　concentration　of　target　PbTx-2　in　the　sample　and　exhibited　a　dynamic　range　from　1.0　pg.mL(-1)　to　10　ng.mL(-1)　with　a　detection　limit　(LOD)　of　0.6　pg.mL(-1)　at　the　3S(blank)　level.　Intra-　and　interassay　coefficients　of　variation　were　below　7.5%　and　9.5%,　respectively.　In　addition,　the　methodology　was　evaluated　for　analysis　of　PbTx-2　in　15　spiked　seafood　samples　and　showed　good　accordance　between　results　obtained　by　the　displacement-type　QCM　immunosensor　and　a　commercialized　enzyme-linked　inununosorbent　assay　(ELISA)　method.