The invention discloses an internal insulating device for a fixed gap overvoltage protector and relates to insulation equipment for overvoltage protectors. The internal insulating device is characterized in that an arrester mandrel is formed by connecting an upper electrode, a resistor disc, a middle electrode and an insulator sequentially from top to bottom; the outside of the arrester mandrel is covered with a composite sheath; a discharge ball I is horizontally mounted on the middle electrode; the insulator is mounted on a non-slip support; a discharge ball II coaxial with the discharge ball I is horizontally mounted on the non-slip support; an insert is arranged in the top of the insulator and is bid end up; the upper portion of the insert is cylindrical, and the lower portion of the insert is a polygonal post; the middle of the polygonal post is provided with a circular groove; the upper end of the insert is provided with a counterbore; the inner wall of the counterbore is provided with an internal thread; the insulator is tightly connected to the non-slip support through a screw in a threaded manner. The internal insulating device has the advantages that the original outer insulator structure is omitted, and the use of the internal insulating structure prevents the fact that the traditional outer insulator easily breaks, easily ages in ultraviolet radiation and has a short running life.