The　integration　of　medium-frequency　transformer　design　and　the　harsh　service　environment　raise　higher　demands　for　the　medium-frequency　insulation　performance　of　epoxy　composites.　As　epoxy　composite＇s　frequency　response　characteristics　and　sensitivity　to　moisture　intrusion　are　still　unknown,　its　application　as　the　insulation　materials　of　medium-frequency　transformers　under　hygrothermal　conditions　is　limited.　This　paper　investigates　the　mechanism　of　moisture　intrusion　damage　to　the　molecular　structure　of　the　epoxy　resin　matrix　and　its　cross-linking　points　with　the　curing　agent.　The　regularity　of　medium-frequency　insulation　performance　degradation　of　epoxy　composites　after　hygrothermal　aging　is　studied.　Results　indicate　that　after　40　days　of　hygrothermal　aging,　moisture　together　with　high　temperature　damaged　the　main　chain　of　the　epoxy　resin　and　its　cross-linking　points　with　the　curing　agent,　resulting　in　a　23　°C　decrease　in　the　glass　transition　temperature　and　an　82　%　decrease　in　cross-linking　density　of　the　epoxy　composite.　Moisture　ionization　and　the　increase　of　carriers　due　to　epoxy　resin　hydrolysis　lead　to　an　increase　in　the　amount　of　space　charge.　The　temperature　rise　caused　by　dipole　steering　polarization　facilitates　the　formation　of　internal　conductive　pathways　in　the　epoxy　composite,　resulting　in　a　73　%　decrease　in　the　medium-frequency　breakdown　strength　and　a　four-order　magnitude　decrease　in　the　medium-frequency　volume　resistivity　of　the　epoxy　composite.　This　study　can　guide　the　insulation　design　of　high-reliability　medium-frequency　transformers.　©　2024