Aiming at the special application requirements of high-voltage submarine cables, this paper proposes a PE-GFRP composite structure and carries out its insulation design. The structure uses glass fiber reinforced plastic (GFRP) as the outer layer of the cable and polyethylene (PE) as the main insulating material in the inner layer. The study systematically analyzed the withstand voltage characteristics of the two materials at different insulation thicknesses and evaluated the leakage current characteristics of the composite structure. In addition, the discharge process of the insulating material is thoroughly investigated through numerical simulation, which visualizes the sprouting and expansion of electric dendrites at different discharge stages and their possible insulation failure mechanisms. The results show that the proposed PE-GFRP composite structure can meet the insulation requirements of high-voltage submarine cables and has good application prospects.