Glass　fiber　reinforce　polymer　(GFRP)　bars　have　better　durability　than　traditional　steel　bars,　but　there　are　few　studies　on　their　long-term　performance　in　actual　service　environments　especially　the　marine　environment.　To　investigate　the　durability　of　GFRP　bars　used　in　cross-sea　bridges,　this　research　delves　into　the　performance　of　132　such　bars,　spanning　diameters　of　10,　12,　16,　and　25　mm,　in　an　environment　that　closely　mimics　real-world　condition.　The　GFRP　bars,　encapsulated　in　concrete,　were　submerged　in　seawater　solution,　maintained　at　either　room　temperature　or　60　degrees　C,　for　periods　spanning　from　0　to　183　days.　Then　the　tensile　strength　and　elastic　modulus　degradation　of　GFRP　bars　were　investigated　by　tensile　tests.　Scanning　electron　microscope　(SEM)　was　also　used　to　analyze　the　micro-degradation　mechanism　of　GFRP　bars.　In　addition,　this　paper　compares　the　four　existing　models　for　predicting　long-term　mechanical　properties　of　GFRP　bars　with　the　experimental　data,　so　as　to　determine　the　formulas　and　methods　to　be　used　in　the　design.　The　result　shows　that　the　elastic　modulus　of　GFRP　bars　after　183　days　of　immersion　in　concrete　pore　solution　was　almost　unchanged　while　the　tensile　strength　decreased　significantly.　Among　them,　the　tensile　strength　of　10　mm　diameter　GFRP　bars　decreased　by　41.52　%,　after　183　days　of　immersion　at　high　temperature　(60　degrees　C).　The　SEM　method　revealed　that　the　main　cause　of　such　phenomena　was　the　weakening　of　the　bond　between　the　fibers　and　matrix,　which　led　to　a　decrease　in　the　interface　property.　Finally,　by　fitting　and　comparing　the　relationship　between　the　test　data　and　the　existing　prediction　models　for　the　long-term　mechanical　properties　of　GFRP　bars,　the　residual　tensile　strength　of　GFRP　bars　used　in　the　actual　project　of　Yanzhou　Bridge　for　100　years　was　predicted　according　to　the　optimal　prediction　model,　i.e.　the　Serb　model,　and　the　results　showed　that　the　residual　strength　of　GFRP　bars　is　able　to　meet　the　current　standard　requirement.