The invention discloses an iron casting digging-replacement welding repair method. The method comprises the following steps that (1) a damaged part of an iron casting is cut off through plasma cutting, and a replacement insert is manufactured through a steel plate according to the shape of the cut part; (2) according to the thickness of the steel plate, a cut iron casting kerf and the edge of thereplacement insert are grooved and ground cleanly, and brand-new base metal is exposed; and (3) carbon dioxide arc welding machine is adopted, power source poles adopt the direct current electrode positive mode, molten pool protection gas is 100% carbon dioxide, a carbon dioxide flux-cored wire is adopted, carbon-dioxide arc welding is conducted at the room temperature, and the replacement insertis connected to the iron casting in an inserted mode. The method is simple in process, convenient to operate, easy in welding and good in fusing performance, through the repair method, the iron casting can be repeatedly used, resources are saved, the ship repair period is shortened, components scrapped due to corrosion and rotting are reduced, cost of shipowners is reduced, and the ship repair cost is reduced.