The invention relates to a clamp nut disassembling tool, in particular to a disassembling device for a clamp nut with a rubber protective sleeve in an electric ring main unit and a cable branch box. The device is characterized in that driven gear shafts distributed in an equilateral triangle shape are arranged in a shell, three driven gears are installed on the driven gear shafts respectively and meshed with a center gear, the three driven gears are provided with convex lugs with center holes towards the outermost end of the center gear, clamping jaws with sharp corners in front ends are arranged in the center holes in the convex lugs, the walls of the convex lugs are provided with positioning screws for adjusting the extending length of the clamping jaws, and an end cover and the center gear are provided with spanner inserting holes. The device has the advantages that operation is easy, working efficiency is high, the clamp nut with the rubber protective sleeve is disassembled with no damage, a large number of electric components are omitted, and the using prospect is good.