The invention relates to a structure of a transformer oil tank conical flange inner hole patching welding copper plate and a welding method, and belongs to the technical field of transformer oil tank manufacturing. The structure has the technical scheme that a copper plate long strip material (3) is in a round shape; the outer diameter dimension is identical to the inner hole minimum inner diameter dimension of a flange; the copper plate long strip material is put in the inner hole of the flange; notches (5) with the same spaces are formed in the copper plate long strip material (3); the notches are longitudinal notches in the width direction of the copper plate long strip material; the length of the notches is 8/10 of the width of the copper plate long strip material; the copper plate between the two adjacent notches is flanged along the circular arc surface of the flange, and is tightly attached onto the circular arc wall position of the inner hole of the flange; the notches and the peripheries of the copper plate long strip material are welded and connected into a whole with the flange. The structure has the advantages that the copper plate is well enabled to be tightly attached onto the inner hole of the flange; the copper plate patched and welded onto the circular arc wall position of the flange can be processed and formed in one step; the operation is simple and convenient; the production efficiency is high; the effect of counteracting heavy current lead wire magnetism leakage by copper plate eddy current reverse magnetic field can be achieved.