Nucleic　acid　nanoswitches　have　a　status　that　cannot　be　ignored　in　the　field　of　biosensing　due　to　the　excellent　biocompatibility　and　flexibility　of　design.　In　our　current　research,　we　have　constructed　a　new　electrochemical　platform　based　on　self-assembled　pH-sensitive　continuous　circular　DNA　nanoswitch　for　miRNA-21　detection.　We　elaborately　designed　an　inside　ring　probe　(IRP)　which　could　form　a　circle　when　complemented　with　an　outside　ring　probe　(ORP).　Under　the　weakly　acidic　condition,　IRPs　and　ORPs　are　self-assembled　into　continuous　annular　DNA,　meanwhile,　the　nanoswitch　is　activated.　However,　if　it　is　not　a　weakly　acidic　environment　with　a　pH　equal　to　6,　these　circles　are　separated　and　the　nanoswitch　cannot　be　triggered.　Therefore,　the　biosensor　doesn＇t　work.　Only　when　the　pH　is　6,　can　the　nanoswitch　be　activated.　Consequently,　a　large　number　of　RuHex　will　accumulate　on　the　continuous　annular　DNA,　which　leads　to　highly　sensitive　detection　of　miRNA-21,　with　concentration　ranged　from　10(-15)　to　10(-8)　M　and　limit　of　detection　down　to　0.84　fM.　More　importantly,　this　nanoswitch-based　biosensor　can　directly　detect　the　target　microRNA　in　human　serum　without　pretreatment.　Therefore,　the　proposed　novel　electrochemical　DNA　nanoswitch　will　have　broad　application　prospects　in　biomarker　detection　and　clinical　diagnosis.