The invention discloses a compressing looseness-proof mechanism for a main column iron core of a shunt reactor. The mechanism comprises a plurality of tensioning screws, a locking plate and a looseness-proof plate, wherein the locking plate is arranged below a main column iron core lower yoke, the looseness-proof plate is arranged below the locking plate, and the plurality of tensioning screws penetrate a disc spring structure, a main column iron core upper yoke, the main column iron core lower yoke, the locking plate and the looseness-proof plate vertically; the disc spring structure is located above the main column iron core upper yoke; a retaining slot is formed in the upper end part of each tensioning screw, symmetrically distributed plane structures are arranged at the lower end part of the tensioning screw, the retaining slot in the tensioning screw extends out of the upper surface of the disc spring structure, a first looseness-proof nut is screwed in a position, below the retaining slot, of the tensioning screw, a looseness-proof washer is mounted in the retaining slot, and a second looseness-proof nut is screwed in a position, above the retaining slot, of the tensioning screw. The mechanism improves the compressing looseness-proof effect on the main column iron core, facilitates maintenance, reduces the maintenance workload, is simple and firm in structure and easy to realize and has good application prospect.