Herein,　we　have　designed　a　unique　oligonucleotide　probe　named　as　bumped　switchable　molecular　probe　(BS-MB)　and　employ　it　to　ultrasensitive　detection　of　miRNA-21.　The　uniqueness　is　that　the　bumped　portion　renders　the　BS-MB　great　design　flexibility　and　divides　its　long　stem　into　two　short　stems　so　that　it　can　play　the　roles　of　recognition　unit,　reporting　unit,　and　polymerization　primer　and　template　simultaneously.　The　opening　of　BS-MB　requires　a　delayed　reaction,　allowing　the　system　a　better-suppressed　background　without　miRNA-21.　In　contrast,　the　introduction　of　miRNA-21　delayed　the　full　opening　of　BS-MB,　inducing　a　concurrent　three-stage　amplification　for　consuming　lots　of　BS-MBs　in　one　amplification　event.　During　the　amplification,　lots　of　target　analogues　of　extended　miRNA-21　(E-miRNA-21)　and　cleavaged　miRNA-21　(C-miRNA-21)　that　can　be　reused　as　miRNA-21　to　cause　further　fluorescence　enhancement　are　repeatedly　generated.　The　system　retains　the　simplicity　(one　oligonucleotide　probe)　and　simplifies　the　detection　procedure　(one-step　detection).　Its　mix-to-signaling　and　oneto-many　amplification　behaviors　are　superior　to　conventional　MB　based　amplification,　exhibiting　a　wide　linear　response　and　a　low　detection　limit　of　8.1　fM.　High　specificity　against　even　one-base　mutation,　desirable　recovery　rates　for　testing　miRNA-spiked　serum　samples,　and　well-defined　accuracy　for　classifying　clinically　related　samples　are　also　demonstrated.