It　remains　technically　challenging　to　develop　a　sensitive　assay　system　to　isothermally　amplify　the　signal　for　miRNA　detection　because　of　its　low　abundance　in　tested　sample,　sequence　similarities　and　existence　in　complex　biological　environments.　In　this　study,　using　miRNA-21　as　target　model,　a　hairpin-inserted　cross-shaped　DNA　nanoprobe　(CP)　with　four　functional　arms　is　constructed　for　the　ultrasensitive　detection　of　miRNA　via　one-step　built-in　target　analogue　(BTA)　cycle-mediated　signal　amplification.　BTA　is　pre-locked　in　one　arm　of　CP　probe　and　inactive.　In　the　presence　of　target　miRNA,　BTA　can　be　unlocked　and　initiate　an　isothermal　amplification　process.　Utilizing　as-designed　CP　probe,　miRNA-21　can　be　detected　to　down　to　500　fM,　and　the　linear　response　range　spans　over　five　orders　of　magnitude.　The　nonspecific　signal　is　less　than　1%　upon　nontarget　miRNAs.　CP　probe　exhibits　~six　times　enhancement　in　resistance　to　nuclease　degradation　and　no　obvious　degradation-induced　fluorescence　change　is　detected　during　the　assay　period.　The　recovery　yield　ranges　from　98.2~105.5%　in　FBS　solution.　Because　of　the　high　sensitivity,　desirable　specificity,　strong　anti-interference　ability　and　substantial　increase　in　nuclease　resistance,　CP　probe　is　a　promising　tool　for　the　detection　of　miRNAs　in　a　complex　biological　milieu.