Glass　Fiber　Reinforced　Plastic　(GFRP)　is　usually　employed　as　the　insulator　material　of　superconducting　cable　terminal,　owing　to　its　excellent　mechanical　properties　and　insulation　strength　at　cryogenic　temperature.　However,　in　the　presence　of　a　void　defect,　both　the　insulation　life　span　and　mechanical　strength　of　GFRP　decrease　significantly　at　superconducting　temperature　conditions.　In　this　paper,　efforts　have　been　made　to　investigate　the　degradation　process　of　GFRP　at　cryogenic　temperature　and　give　insight　into　its　degradation　mechanism.　By　comparing　PD　activities　at　278.7　K,　223　K　and　153　K　in　initial　phase　of　aging,　it　is　found　that　PDs　are　inclined　to　be　inhibited　at　lower　temperature.　Experiments　indicate　that　the　deposited　charge　accumulation　and　mechanical　properties　degradation,　instead　of　chemical　aging　and　PD　energy,　play　a　crucial　part　in　dielectric　degradation.　At　cryogenic　temperatures,　the　local　heat　accumulation　brought　by　PDs　increases　the　material　internal　mechanical　stress,　and　the　accumulation　of　deposited　charges　causes　instantaneous　energy　release.　On　the　basis　of　the　decrease　of　the　mechanical　properties　of　the　material,　the　crack　expands　rapidly,　which　leads　to　the　dielectric　failure　of　the　material.　In　conclusion,　the　degradation　of　GFRP　at　cryogenic　temperature　is　the　result　of　the　combined　effects　of　deposited　charge　accumulation　and　mechanical　properties　decline.