Exploiting　new　sensing　mechanism　for　multiple　signal　responses　has　become　an　emerging　research　trend　of　gas-sensitive　photoelectrochemical　(PEC)　immunoassay.　Herein,　we　design　a　double-mode　visual　and　PEC　immunosensing　system　for　the　sensitive　detection　of　prostate-specific　antigen　(PSA,　as　a　model　analyte)　on　the　basis　of　gasochromic　reaction　with　Pd-MoO3.　As　the　sandwich　immunoreaction　progresses,　a　platinum　nanoparticle-labeled　detection　antibody　catalyzes　the　production　of　H-2　from　the　substrate　(NH3BH3).　The　resulting　H-2　undergoes　a　gasochromic　reaction　with　Pd-MoO3,　resulting　in　a　change　in　color　of　the　Pd-MoO3-coated　electrode　to　provide　a　visual　readout.　Meanwhile,　the　photocurrent　intensity　of　the　product　increases　due　to　the　reduction　of　the　bandgap,　which　is　positively　related　to　the　concentration　of　the　analyte.　Under　optimal　conditions,　the　developed　sensing　platform　presented　a　high　sensitivity　for　PSA　in　a　dynamic　working　range　of　0.1-50　ng　mL(-1)　with　a　limit　of　detection　of　76　pg　mL(-1).　Impressively,　this　work　not　only　presents　a　new　protocol　for　the　application　of　gasochromic　materials　but　also　gives　a　path　for　the　developing　dual　signal　readout　in　gas-sensitive　PEC　immunoassay.