Exosome-based　liquid　biopsies　highlight　potential　utility　in　diagnosis　and　determining　the　prognosis　of　patients　with　cancer　and　other　diseases.　However,　the　existing　techniques　are　severely　limited　for　practical　applications　due　to　the　complications　of　high　cost,　low　sensitivity,　tedious　procedures,　and　large　sample　consumption.　Herein,　we　report　a　microstructured　optical　fiber　sensor　for　fast,　sensitive,　and　accurate　quantification　of　exosomes　in　blood　samples　of　breast　cancer　patients.　Numerical　simulations　are　applied　to　demonstrate　that　hollow-core　microstructured　anti-resonant　fibers　(HARFs)　can　stringently　confine　light　in　the　fiber　core,　ensuring　strong　light-matter　interaction　and　thus　maximumly　amplifying　the　signal.　Taking　this　advantage,　a　AuNPs-dsDNA　assembly　containing　gold　nanoparticles,　a　recognizing　DNA　aptamer,　and　a　fluorescent　reporter　DNA　sequence　is　fabricated　followed　by　immobilization　on　the　fiber　wall　to　form　a　AuNPs-　dsDNA-HARF　sensor.　Cancer-derived　exosomes　can　be　recognized　and　captured　in　the　fiber　channel　and　generate　dose-dependent　fluorescent　signals　for　quantification.　The　microfiber　sensor　demonstrates　enhanced　sensitivity　and　specificity,　enabling　the　detection　of　single　digits　of　exosome　particles　at　the　nanoliter　sample　level.　In　addition,　by　tracking　exosome　phenotypic　changes,　the　proposed　fiber　sensor　can　facilitate　precise　drug　treatment　monitoring.　This　work　provides　a　robust　platform　for　exosome-based　biopsy　for　cancer　diagnosis　and　prediction　of　therapeutic　outcomes.