A　new　photoelectrochemical　(PEC)　immunosensor　based　on　Mn-doped　CdS　quantum　dots　(CdS:Mn　QDs)　on　g-C3N4　nanosheets　was　developed　for　the　sensitive　detection　of　prostate　specific　antibody　(PSA)　in　biological　fluids.　The　signal　derived　from　the　as-synthesized　Cd:Mn　QDs-functionalized　g-C3N4　nanohybrids　via　a　hydrothermal　method　and　was　amplified　through　DNAzyme　concatamers　on　gold　nanoparticles　accompanying　enzymatic　biocatalytic　precipitation.　Experimental　results　by　UV-vis　absorption　spectra　and　photoluminescence　revealed　that　CdS:Mn　QDs/g-C3N4　nanohybrids　exhibited　higher　photocurrent　than　those　of　CdS:Mn　QDs　and　g-C3N4　alone.　Upon　addition　of　target　PSA,　a　sandwich-type　immunoreaction　was　carried　out　between　capture　antibodies　and　the　labeled　detection　antibodies.　Accompanying　introduction　of　gold　nanoparticles,　the　labeled　initiator　strands　on　the　AuNPs　triggered　hybridization　chain　reaction　and　the　formation　of　DNAzyme　concatamers　in　the　presence　of　hemin.　The　formed　DNAzyme　catalyzed　4-chloro-1-naphthol　(4-CN)　to　produce　an　insoluble/insulating　precipitate　on　the　Mn:CdS　QDs/g-C3N4,　and　blocked　the　light　harvesting　of　Mn:CdS　QDs/g-C3N4,　thus　resulting　in　the　decreasing　photocurrent.　Under　optimal　conditions,　the　immunosensor　exhibited　good　photocurrent　responses　for　determination　of　target　PSA,　and　allowed　detection　of　PSA　at　a　concentration　as　low　as　3.8　pg　mL(-1).　The　specificity,　reproducibility　and　precision　of　this　system　were　acceptable.　Significantly,　this　methodology　was　further　evaluated　for　analyzing　human　serum　samples,　giving　well-matched　results　with　referenced　PSA　enzyme-linked　immunosorbent　assay　(ELISA)　method.