The invention relates to the technical field of transmission lines, and discloses an on-site fast corrosion prevention treatment method for a transmission line grounding downlead. The on-site fast corrosion prevention treatment method comprises the steps of (1) corrosion prevention protection, that is, the surface of the grounding downlead is coated with a layer of protective material, then the protective material is externally provided with an outer sheath in a sleeved manner; (2) grounding, that is, the bottom part of the grounding downlead is buried underground, and back-filling and fixation are performed by soil doped with dry quicklime, and compaction is carried out; and (3) membrane laying, that is, a waterproof membrane is laid on the back-filling soil, and four corners of the waterproof membrane are fixed by the soil. The on-site fast corrosion prevention treatment method sets about from the root cause of easy corrosion of the grounding downlead, can effectively reduce the corrosion rate of the grounding downlead, and thus significantly prolonging the service life of the grounding downlead. In addition, the grounding downlead can perform corrosion prevention treatment on site, and is fast and efficient.