The　microRNAs　(miRNAs)　play　a　critical　role　in　many　biological　processes　and　are　essential　biomarkers　for　diagnosing　disease.　However,　the　sensitive　and　specific　quantification　of　microRNAs　(miRNAs)　expression　in　living　cells　still　faces　a　huge　challenge.　Our　study　designed　a　multifunctional　linear　DNA　nanostructure　(MLN)　as　a　carrier　of　molecular　beacons　(MB-21)　for　detecting　and　intracellular　imaging　miRNA-21.　The　MLN-MB　consists　of　three　parts:　aptamer,　MLN,　and　MB-21.　The　aptamer　(AS1411)　could　media　MLN-MB　enter　live　cells　without　additional　transfection　reagents.　Once　inside　the　cells,　the　intracellular　miRNA-21　could　hybridize　the　MB-21s,　resulting　in　significantly　enhanced　fluorescence　signals.　The　whole　process　was　enzyme-free,　autonomous,　and　continuous,　which　avoided　the　necessity　of　adding　external　fuel　strands　or　enzymes.　We　demonstrated　that　the　MLN-MB　could　be　used　to　screen　the　miRNA-21　with　a　detection　limit　of　320　pM　in　a　short　time　(10　min)　and　show　high　specificity　toward　miRNA-21　against　other　miRNAs.　Moreover,　the　proposed　MLN-MB　could　detect　the　miRNA-21　in　complex　matrixes　stably.　With　its　outstanding　stability,　dual　recognition,　and　biocompatibility,　MLN-MB　is　capable　of　delivering　into　living　cells　to　identify　specific　cancer　cells.　Therefore,　our　sensing　approach,　with　high　sensitivity,　specificity,　and　simplicity　advantages,　holds　great　potential　for　early　cancer　diagnosis.