The invention discloses an anti-theft device for a 35 kV outdoor pole-climbing platform support. The anti-theft device comprises an anti-theft sheath, a theft-proof nut and a fastening tool, wherein the anti-theft sheath is hollow and cylindrical, and threads are formed on the internal surface of the anti-theft sheath; the theft-proof nut comprises a hollow cylindrical inner core, a pentagonal outer sleeve and a flange, the pentagonal outer sleeve and the flange are fixed outside the cylindrical inner core, the top of the flange is fixed together with the bottom of the pentagonal outer sleeve, the theft-proof nut adopts an integrally formed structure, and threads are formed on the internal surface of the theft-proof nut; the fastening tool comprises a cylindrical sleeve, the lower part of the internal surface of the cylindrical sleeve takes the shape of a pentagon matched with the pentagonal outer sleeve of the theft-proof nut, and the lower part of the cylindrical sleeve sleeves the pentagonal outer sleeve of the theft-proof nut. The anti-theft device for the 35 kV outdoor pole-climbing platform support is simple in structure, and convenient to produce and mount, and the security is improved to a certain extent while an original hexagonal nut is not damaged.