MicroRNAs　play　a　crucial　role　in　regulating　gene　expression　and　cellular　function.　Reliable　detection　of　miRNA　is　highly　demanded　in　clinical　diagnosis　and　therapy.　Herein,　we　designed　a　structure-convertible　DNA　switch　and　constructed　a　novel　switch-conversional　ratiometric　fluorescence　biosensor　(SCRF　biosensor)　for　highly　sensitive　miRNA　detection　by　the　use　of　amplicon　fragments　to　convert　the　structure　of　the　switch.　The　DNA　switch　was　a　sophisticated　designed　single-strand　DNA　with　a　stem-loop　structure　and　modified　with　two　fluorophores　(Cy3　and　Cy5)　and　one　quencher　at　specific　sites　of　the　switch.　Amplicon　fragments　(c*)　were　produced　by　an　exponential　amplification　reaction.　When　the　c*　hybridized　to　the　loop　of　a　DNA　switch,　the　structure　of　the　switch　would　convert,　and　fluorescence　resonance　energy　transfer　occurred　between　Cy5　and　Cy3.　Then　two　fluorescence　signals　with　different　trends　would　be　observed.　As　a　result,　by　the　ratio　of　the　two　signals,　we　can　quantitatively　and　quickly　detect　the　target　miRNA　with　the　concentration　range　from　100　fM　to　100　nM　and　the　excellent　detection　limit　down　to　70.9　fM,　providing　this　new　SCRF　biosensor　broad　application　prospects.