Abstract: Epoxy resins are susceptible to be eroded by hydrothermal environment, which causes deterioration of insulation performance, and directly impairs the safety and stability of power system operation. In order to clarify the change trend of the dielectric properties of epoxy resin materials in hydrothermal ambience, artificial hydrothermal aging experiments were conducted on cured epoxy resin samples to investigate the effect of aging temperature on its electrical properties, and the aging mechanism was analyzed. The experimental results indicate that the mass of epoxy resin samples will augment due to moisture absorption after hydrothermal aging. Plasticizing effect of water molecules on the epoxy resin matrix will lower its glass transition temperature. The volume resistivity of epoxy resin samples will decrease after exposing to hydrothermal ambience, while the relative permittivity and dielectric loss factor will increase especially in the low frequency region. The AC breakdown phenomenon of epoxy resin samples is explained using Wagner's thermal breakdown theory, and the power frequency AC breakdown strength of epoxy resin will decrease due to the increase in relative permittivity and dielectric loss factor. Higher aging temperature can promote the hygroscopic process of the epoxy resin, which will increase the moisture content in equilibrium state and cause the dielectric properties to further decrease.