Lung　cancer　with　worldwide　distribution,　high　incidence　and　low　survival　rate　is　significantly　and　increasingly　threatening　human　health.　Thus,　the　specific　detection　of　lung　cancer-associated　biomarkers　is　of　crucial　importance　in　early　diagnosis　and　treatment.　In　this　work,　taking　microRNA-21　as　an　example,　a　biosensor　is　proposed　via　a　stimuli-induced　strand　displacement　amplification　(SDA)　and　cascade　signal　amplification　with　the　assistance　of　polymerase.　An　allosteric　molecular　beacon　(MB)　with　chemical　modification　is　designed　to　emit　the　enhanced　fluorescent　signal　in　presence　of　microRNA　target.　The　sensing　system　possesses　a　linear　calibration　curve　from　5　pM　to　40　nM　with　the　limit　of　detection　(LOD)　of　0.7　pM　and　displays　good　specificity　to　discriminate　coexisting　microRNAs.　In　addition,　the　feasibility　is　confirmed　by　performing　the　detection　of　miRNA-21　extracted　from　non-small　cell　lung　cancer　(NSCLC)　A549　cells,　and　a　good　recovery　is　achieved　in　complex　human　serum　sample.　Therefore,　the　miRNA-triggered　cascaded　amplification　would　be　crucial　strategy　to　facilitate　microRNA　analysis　in　the　biological　detection　and　broad　clinical　applications.