An offline ladder stabilizing device for a 500 kV transmission line comprises a brace and a seat post connecting sleeve assembly, wherein a clamping device is mounted at each of two ends of the brace and comprises a handle, a screw rod, a clamping hook, a clamping press block and a rotating shaft, the screw rod is mounted on one side of the clamping hook, the handle is mounted at one end of the screw rod, the clamping press block is mounted at the other end of the screw rod, and the rotating shaft is mounted between the screw rod and the clamping press block; the seat post connecting sleeve assembly is arranged in the middle of the brace in a sleeving manner. The offline ladder stabilizing device has the advantages that two vertical rods of a ladder can be fixed by the two clamping device, original two-point fixation of the ladder can be changed to multi-point fixation by two seat posts, so that the stability of the ladder is improved greatly, the ladder is prevented from shaking more violently when a worker climbs the ladder, and the safety of the worker can be guaranteed.