MicroRNA-21(miRNA-21)　can　act　as　a　novel　biomarker　in　early　stage　of　acute　kidney　injury　(AKI).　The　development　of　simple　but　sensitive　miRNA-21　detection　technology　is　helpful　for　prevention　and　treatment　of　AKI.　In　this　work,　based　on　the　cascaded　signal　amplifications　of　the　target-induced　hybridization　chain　reaction　(HCR)　and　magnetic　assisted　enrichment,　a　highly　sensitive　homogeneous　electrochemiluminescence　(ECL)　biosensor　for　miRNA-21　detection　had　been　developed.　Firstly,　capture　probe　H1　(CPH1)　had　been　modified　on　magnetic　beads　(MBs)　which　could　capture　target　(miRNA-21)　and　trigger　HCR　amplification　to　produce　long　strand　dsDNA　in　the　homogeneous　solution.　Ru(phen)32+　could　be　embedded　into　the　grooves　of　dsDNA　and　played　the　role　of　luminous　tag,　which　could　be　separated　by　magnetic　easily　to　realize　the　enrichment.　Then　the　enriched　products　had　been　dissolved　in　the　buffer　solution　and　the　ECL　signal　could　be　detected　easily.　The　ECL　intensity　had　a　linear　correlation　with　the　logarithm　of　miRNA-21　concentration　in　the　range　of　1.0　fM-1.0　nM,　the　detection　limit　was　0.14　fM.　The　method　developed　in　this　work　showed　well　sensitivity,　specificity,　which　had　been　applied　to　detect　miRNA-21　in　urine　of　AKI　patients　with　satisfied　results.