The utility model discloses a fixed-gap overvoltage protector and relates to the voltage protector insulating equipment. The fixed-gap overvoltage protector comprises a lightning arrester core rod which is composed of an upper electrode, a resistor disc, a middle electrode and an insulator all connected orderly from top to bottom; a compound outer sleeve is arranged to cover the lightning arrester core rod; a discharge ball I is horizontally mounted on the middle electrode; the insulator is mounted on an anti-slip support; a discharge ball II coaxial with the discharge ball I is horizontally mounted on the anti-slip support; an insert is arranged in the top of the insulator; the insert is wide in top and narrow in bottom; the upper part of the insert is cylindrical, while the lower part of the insert is polyhedral and cylindrical; an annular groove is circularly formed in the middle section of the polyhedral and cylindrical lower part of the insert; a counter bore is formed in the upper end of the insert; internal threads are formed on the inner wall of the counter bore; the insulator is tightly connected to the anti-slip support by use of a screw and the threads. The built-in insulating device of the fixed-gap overvoltage protector has the advantages that the built-in insulating structure is adopted instead of the original external insulator structure, and therefore, the problem that the existing external insulator is prone to undergo breakage and ultraviolet radiation aging, and quickly drops out of service is solved.