The invention relates to a device for replacing a main material of a transmission tower. The transmission tower comprises a longitudinally arranged tower body and a cross arm transversely arranged on the tower body, wherein the tower body is provided with a tower main segment for fixing the transmission tower, and the tower main segment is formed by two main segment steel angles which are oppositely arranged and is rolled steel with a cross-shaped section. The replacing device comprises a middle structure section. The device for replacing the main material of the transmission tower can be used for replacing a damaged main material of the tower and has the advantages of construction simplicity, transportation convenience and no site limitation.