Herein,　a　portable　photoelectrochemical　(PEC)　in-vitro　diagnostic　platform　for　the　ultrasensitive　detection　of　alpha-fetoprotein　(AFP)　based　on　the　cadmium　sulfide　(g-O-CdS)　photoelectrode　with　graded　oxygen　doping　was　constructed.　An　LED　flashlight　is　used　as　the　light　source,　and　a　digital　multimeter　(DMM)　is　used　as　the　signal　readout　device　to　achieve　point-of-care　testing　(POCT).　Due　to　the　spatially　graded　concentration　of　oxygen　doping,　the　internal　built-in　band　bending　enhanced　the　separation　of　photogenerated　electron-hole　pairs　and　facilitated　photocorrosion　from　the　oxidation　of　S　atoms　by　the　internal　holes,　and　the　intensity　and　stability　of　the　photocurrent　increased　significantly.　Quantitative　analysis　of　AFP　was　realized　by　introducing　H2O2　(photoanode　sacrificial　agent)　into　the　detection　system　through　sandwich　immunoreaction　and　the　modification　of　glucose　oxidase　(GOx)　on　the　detection　antibody　and　using　DMM　readout　for　the　change　of　photocurrent　signal.　Under　optimal　conditions,　the　photocurrent　was　positively　correlated　with　the　AFP　concentration　in　the　dynamic　working　range　from　0.2　to　100　ng　mL-1　with　a　limit　of　detection　of　74.8　pg　mL-1.　Such　a　sensitive,　stable,　and　reliable　PEC　immunoassay　miniaturizes　the　equipment　and　eliminates　the　need　for　expensive　instrumentation,　thereby　offering　a　new　avenue　for　the　development　of　user-friendly　and　low-cost　assay　protocol　for　practical　utilization　in　the　quantitation　of　low-abundance　analyte.