Abstract: External factors such as bedrock wear, ship anchor damage, and sheath corrosion can lead to direct exposure of submarine cable insulation to the seawater environment, seriously affecting the safe and stable operation of submarine cables. This article takes cross-linked polyethylene (XLPE) insulation as the research object to study the influence of seawater environments on the insulation performance of XLPE. Firstly, an accelerated seawater corrosion test platform was established, and XLPE slices were subjected to accelerated seawater corrosion experiments at different time. Secondly, the impact of seawater on the breakdown characteristics of the sample was analyzed through power frequency breakdown voltage experiments. Thirdly, the physical and chemical properties of insulation materials were analyzed through Fourier transform infrared spectroscopy and differential scanning calorimetry experiments. Finally, a molecular dynamics model for seawater corrosion of XLPE insulation was constructed, and the corrosion mechanism of XLPE insulation materials in seawater environment was revealed based on experimental results. The results show that, due to the recrystallization caused by high temperatures, the crystallinity of XLPE samples will first increase slightly and then gradually decrease under the continuous actions of seawater, resulting in an increase and then a decrease in the breakdown voltage. As the corrosion time of seawater increases, the permeation of seawater causes partial chemical bond breakage and molecular chain disruption in the XLPE system, and the band gap of the XLPE system decreases, resulting in a decrease in the energy required for electrons to transition from the valence band to the conduction band. This is also an important reason for the decline in the insulation performance of XLPE. Finally, based on the molecular dynamics simulation and experimental results of seawater corrosion of XLPE, a prediction model for the breakdown field strength of cross-linked polyethylene in seawater corrosion considering temperature and time is proposed. Based on this prediction model, the breakdown field strength of cross-linked polyethylene in seawater environment can be effectively predicted. The research results have important reference value for revealing the failure mechanism and life prediction of cross-linked polyethylene insulation in extreme underwater environments.