In　this　work,　we　present　a　highly　sensitive　photoelectrochemical　(PEC)　sensing　platform　for　malathion　(Mal)　detection　based　on　the　biocatalysis-induced　formation　of　BiOBr/Bi2S3　semiconductor　heterostructures　(BIFBSH).　Initially,　a　double-stranded　DNA　consisting　of　an　initiator　strand　and　the　anti-Mal　aptamer　was　immobilized　on　magnetic　beads　surface.　The　target　Mal　could　competitively　bind　with　its　aptamer　and　liberate　the　initiator　strand　from　the　double-stranded　DNA.　Subsequently,　the　initiator　strand　triggered　a　hybridization　chain　reaction　(HCR)　to　produce　a　long　DNA　concatemer　containing　the　sequence　of　G-quadruplex　structure.　The　DNA　concatemer　was　then　endowed　with　HRP-like　activity　via　the　formation　of　hemin/G-quadruplex　complexes　and　binding　with　MnTMPyP.　The　HCR-synthesized　HRP-mimicking　DNA　concatemer　(HSHMDC)　was　employed　to　catalyze　Na2S2O3　conversion　to　H2S　with　the　aid　of　H2O2.　The　generated　H2S　could　react　with　BiOBr　nanoflowers　(BiOBrNFs)　modified　on　an　indium　tin　oxide　electrode,　resulting　in　the　in　situ　formation　of　BiOBr/Bi2S3　heterostructures　with　enhanced　photoelectric　conversion　efficiency.　The　BiOBr/Bi2S3　heterostructures　could　generate　a　strong　photocurrent　signal　to　indicate　Mal　recognition.　Under　the　optimal　conditions,　a　wide　linear　range　between　the　Mal　concentration　and　photocurrent　intensity　(0.001–1000　ng　mL−1)　and　a　limit　of　detection　(LOD)　as　low　as　0.12　pg　mL−1　were　achieved.　©　2021　Elsevier　B.V.