The invention relates to a theftproof nut for a high voltage tower, and particularly relates to a round compound theftproof nut. The round compound theftproof nut comprises a nut body which is characterized in that the lower part of the nut body is of a round structure, a prismatic structure is raised outward on the upper part, a screw hole is formed in the middle, an arc-shaped theftproof slot is formed in the middle of the screw hole, a baffle block is mounted in the theftproof slot, a spring is mounted on the baffle block, a steel ball is arranged at the other end of the spring, and a torque groove, the diameter of which is greater than the diameter of the screw hole, is formed in the middle part of the prismatic structure. The round compound theftproof nut of the structure has the advantages that the round compound theftproof nut is simple in structure, and is good in anti-loose and theftproof effects as the nut cannot be detached by an external force.