In　recent　years,　more　and　more　research　is　being　conducted　on　microRNAs　and　their　involvement　in　the　regulation　of　autophagy　phagocytosis　which　is　closely　related　to　tumor　growth.　MicroRNA-21　is　a　kind　of　small　RNA　that　can　regulate　gene　expression　and　plays　a　significant　role　in　autophagy　of　tumor　cells.　But　the　detection　of　microRNAs　had　always　been　a　problem　in　the　field　of　biological　analysis.　In　this　study,　we　designed　a　new　fluorescent　sensor　for　the　detection　of　miRNA-21.　The　sensor　was　based　on　the　successful　signal　reporting　by　E36-encapsulated　vesicles　and　the　specific　interaction　between　E36　and　miRNA-21.　In　the　presence　of　miRNA,　the　E36/miRNA-21　complex　formed　and　served　as　a　donor　molecule　inside　the　acceptor　PDA　vesicles　to　amplify　the　fluorescence　through　FRET.　Additionally,　the　sensor　was　applied　to　detect　miRNA-21　in　complex　biological　samples　with　satisfactory　results.