Photocurrent-polarity　conversion　strategies　are　typically　realized　by　constructing　complex　photovoltaic　electrodes　or　changing　the　relevant　conditions,　but　most　involve　poor　photogenerated　carrier　transfer　efficiency　and　cumbersome　experimental　steps.　To　this　end,　a　photoelectrochemical　(PEC)　biosensor　by　utilizing　ascorbic　acid　(AA)-induced　photocurrent-polarity-switching　was　proposed　for　the　detection　of　carcinoembryonic　antigen　(CEA).　Under　light　excitation,　the　electron　donor　AA　was　oxidized　by　the　photogenerated　holes　of　photoactive　material　Co-NC/CdS,　resulting　in　the　conversion　of　cathodic　photocurrent　to　the　anodic　direction.　In　the　presence　of　the　target　CEA,　alkaline　phosphatase　(ALP)　was　introduced　into　the　microplates　by　the　sandwiched　immunoreaction,　which　then　catalyzed　the　production　of　AA　from　ascorbic　acid-2-phosphate　(AAP).　Finally,　the　catalytic　product　AA　was　transferred　onto　Co-NC/CdS-modified　screen-printed　carbon　electrode,　thus　activating　photocurrent-polarity-switching　platform.　The　anodic　photocurrent　values　gradually　increased　with　increasing　CEA　concentration　in　the　range　of　0.02-80　ng　mL(-1)　and　reached　a　limit　of　detection　(LOD)　of　8.47　pg　mL(-1)　(S/N　=　3).　In　addition,　the　results　of　actual　sample　detection　prove　the　reliability　of　the　constructed　PEC　biosensor.　Importantly,　this　work　relies　on　a　mobile　smartphone　wireless　Bluetooth　device　coupled　with　the　PEC　biosensor　for　immediate　detection,　providing　another　idea　for　detecting　CEA　in　clinical　diagnosis.