A　gold　nanoparticle　(AuNPs)/gallium　nitride　(GaN)　Schottky　junction　was　fabricated　by　growing　AuNPs　in　situ　on　the　surface　of　GaN　and　then　etched　by　H2O2　to　appropriate　diameter.　The　photogenerated　electrons　of　GaN　can　be　captured　and　transferred　by　the　AuNPs　to　increase　the　migration　efficiency,　and　the　electron-hole　pairs　were　separated,　which　results　in　the　enhancement　of　the　photoelectric　performance　of　the　system.　The　Fermi　energy　level　of　AuNPs　and　the　charge　transfer　efficiency　of　the　AuNPs/GaN　can　be　adjusted　by　controlling　the　size　of　the　AuNPs.　Then　the　AuNPs/GaN　Schottky　photoelectrode　had　been　applied　to　develop　a　novel　photoelectrochemical　(PEC)　aptasensor　for　the　epithelial　ovarian　cancer　marker-CA125　detection.　The　DNA　aptamer　of　CA125　was　modified　on　the　surface　of　the　AuNPs　via　Au-S　bonds.　The　aptamer　can　bind　with　the　target　with　high　selectivity,　and　the　photoelectron　transfer　process　of　the　system　can　be　blocked　by　the　protein,　which　results　in　the　decrease　of　the　photocurrent　of　the　system.　The　ratio　of　photocurrent　before　and　after　incubation　with　CA125　(I-1/I-0)　has　a　linear　with　the　concentration　of　CA125　in　the　range　of　1-100　U/mL　with　a　detection　limit　of　0.3　U/mL.　The　standard　addition　recovery　rates　were　between　86.01%　and　90.09%.　This　method　showed　good　sensitivity,　selectivity,　and　reliability　in　detecting　CA125　in　serum.