Exosomes　that　carry　large　amounts　of　tumor-specific　molecular　information　have　been　identified　as　a　potential　non-invasive　biomarker　for　early　warning　of　cancer.　In　this　work,　we　reported　an　enzyme-assisted　photoelectrochemical　(PEC)　biosensor　for　quantification　of　exosomes　based　on　the　in　situ　synthesis　of　Ti3C2　MXene/CdS　composites　with　magnetic　separation　technology　and　hybridization　chain　reaction　(HCR).　First,　exosomes　were　specifically　bound　between　aptamer-labeled　magnetic　beads　(CD63-MBs)　and　a　cholesterol-labeled　DNA　anchor.　The　properly　designed　anchor　ends　acted　as　a　trigger　to　enrich　the　alkaline　phosphatase　(ALP)　through　HCR.　It　catalyzed　more　sodium　thiophosphate　to　generate　the　sulfideion　(S2−),　which　combined　with　Cd2+　for　in　situ　fabrication　of　CdS　on　Ti3C2　MXene　resulting　in　elevated　photocurrent.　The　Ti3C2　MXene-anchored　PEC　method　was　realized　for　the　quantitative　detection　of　exosomes,　which　exhibited　the　dynamic　working　range　from　7.3　×　105　particles　per　mL　to　3.285　×　108　particles　per　mL　with　a　limit　of　detection　of　7.875　×　104　particles　per　mL.　The　strategy　showed　acceptable　stability,　high　sensitivity,　rapid　response　and　excellent　selectivity.　Furthermore,　we　believe　that　the　PEC　biosensor　has　huge　potential　as　a　routine　bioassay　method　for　the　precise　quantification　of　exosomes　from　breast　cancer　in　the　future.　©　2022　The　Royal　Society　of　Chemistry.