The　abnormal　expression　of　miRNA-21　is　often　found　in　tumor　specimens　and　cell　lines,　and　thus,　its　specific　detection　is　an　urgent　need　for　the　diagnosis　and　effective　therapy　of　cancers.　In　this　contribution,　we　demonstrate　a　palindrome-based　hybridization　chain　reaction　(PHCR)　upon　the　stimuli　of　a　short　oligonucleotide　trigger　to　perform　the　autonomous　assembly　of　cross-linked　network　structures　(CNSs)　for　the　amplification　detection　of　miRNA-21　and　sensitive　fluorescence　imaging　of　cancerous　cells.　The　building　blocks　are　only　two　palindromic　hairpin-type　DNA　strands　that　are　separately　modified　with　different　fluorophores　(Cy3　and　Cy5),　which　is　easily　combined　with　the　catalytic　hairpin　assembly　(CHA)　technique　that　can　further　amplify　the　signal　output.　Utilizing　the　CHA-PHCR　assay　system,　a　small　amount　of　miRNA-21　can　activate　many　triggers　via　CHA　and　in　turn　induce　the　PHCR-based　CNS　assembly　from　more　DNA　building　blocks,　bringing　Cy3　and　Cy5　into　close　proximity　to　each　other　and　generating　ultrasensitive　fluorescence　resonance　energy　transfer　signals.　As　a　result,　target　miRNA　can　be　quantitatively　detected　down　to　as　low　as　10　pM　with　high　assay　specificity.　The　coexisting　nontarget　miRNAs　and　other　biomacromolecules　do　not　interfere　with　signal　transduction.　The　developed　assay　system　is　suitable　for　screening　different　expression　levels　of　miRNA-21　in　living　cells　by　fluorescence　imaging.　The　palindrome-based　cross-linking　assembly　can　enhance　the　intracellular　stability　of　assembled　nanostructures　by　at　least　fivefold　and　exhibit　the　good　universality　for　the　detection　of　other　miRNAs.　Moreover,　cancerous　cells　can　be　distinguished　from　healthy　cells,　and　the　CHA-PHCR　assay　is　in　good　accordance　with　the　gold　standard　PCR　method,　indicating　a　promising　platform　for　the　diagnosis　of　human　cancers　and　other　genetic　diseases.