Electrical　tree　degradation　is　one　of　the　main　causes　of　insulation　failure　in　high-frequency　transformers.　Electrical　tree　degradation　is　studied　on　pure　epoxy　resin　(EP)　and　MgO/EP　composites　at　frequencies　ranging　from　50　Hz　to　130　kHz.　The　results　show　that　the　tree　initiation　voltage　of　EP　decreases,　while　the　growth　rate　and　the　expansion　coefficient　increase　with　frequency.　Moreover,　the　bubble　phenomenon　at　high　frequencies　in　EP　composites　is　discussed.　Combined　with　trap　distribution　characteristics　within　the　material,　the　intrinsic　mechanism　of　epoxy　composites　to　inhibit　the　growth　of　the　electrical　tree　at　different　frequencies　is　discussed.　It　can　be　concluded　that　more　deep　traps　and　blocking　effect　are　introduced　by　doping　nano-MgO　into　EP　bulks,　which　can　improve　the　electrical　tree　resistance　performance　of　EP　composites　in　a　wide　frequency　range.