Highly　sensitive　detection　of　biomarkers　is　essential　for　disease　prevention　and　early　diagnosis.　Herein,　a　highly　sensitive　strategy　was　proposed　for　microRNA-21　(miRNA-21)　detection　by　the　incorporation　of　programmable　toehold-mediated　strand　displacement　(TMSD)　and　dark-field　microscopy　imaging.　Firstly,　efficient　and　specific　TSMD　was　carried　out　via　hybridization　between　the　substrate　strand　(Sub)　and　two　short　probe　strands　(P1,　P2).　Then,　miRNA-21　could　specifically　hybridize　with　Sub　due　to　the　toehold　that　existed　on　its　tail,　which　triggered　the　amplification　with　the　help　of　the　assist　strands,　and　forming　a　large　number　of　Sub-assist　double-stranded　DNA　(dsDNA).　This　process　realized　the　targeted　highly　specific　recognition　of　miRNA-21　and　the　amplification　of　the　trace　target　to　high-output　dsDNA.　Additionally,　as　glucose　oxidase　(Gox)　was　modified　on　the　end　of　the　assist　strands　in　advance,　hydrogen　peroxide　was　generated　after　adding　glucose　to　the　system,　which　further　etched　gold-silver　core-shell　nanocubes　(Au@Ag　NCs).　As　a　result,　the　size　of　Au@Ag　NCs　decreased　and　the　scattering　intensity　reduced　simultaneously.　The　scattering　intensity　reduction　value　of　Au@Ag　NCs　has　a　linear　relationship　with　miRNA-21　concentration　in　the　range　of　1.0　to　100.0　fM　with　a　limit　of　detection　of　1.0　fM.　Finally,　the　proposed　method　has　been　successfully　demonstrated　for　the　determination　of　miRNA-21　in　lung　cancer　cell　A549　lysate.