The invention discloses an insulation structure of an extra-high-voltage open type transformer substation current transformer, and belongs to the technical field of high-voltage electrical equipment. The insulation structure comprises a base, a shell and a composite insulating sleeve used for connecting the base and the shell, wherein a center conductor is arranged in the shell and sequentially sleeved with an inner shielding cylinder and an outer shielding cylinder, a secondary line shielding tube extending from the bottom of the outer shielding cylinder extends into the composite insulating sleeve and is connected to the base, and two-level shielding units used for voltage reduction are further arranged on the secondary line shielding tube. The insulation structure is inverted, and the center conductor, the inner shielding cylinder and the outer shielding cylinder are arranged in the shell at the top of the current transformer, so that electric field distribution in a high-voltage part is more uniform; the secondary line shielding tube and a voltage reduction structure are arranged in the composite insulating sleeve, and the two-level shielding unit structure is adopted for the voltage reduction structure, so that each layer of distribution bears corresponding high voltage, the electric field is reduced level by level, and the purpose of voltage partition is achieved. The insulation structure is simple and easy to assemble.