The invention discloses a reduction casting compositing method and device for large iron ingots. The method includes: selecting a core blank, adjusting the temperature of the core blank, pre-descaling the surface of the core blank, reducing the oxide scale on the surface of the core blank, casting and compositing, cooling the composite ingot, and repeatedly casting and compositing the composite ingot. The device used by the method comprises a furnace, a shell, a thermal insulation layer, an inner shell, an exhaust port, a vacuum air extractor, an inert gas inlet, a steel ingot die, a sizing block, the core blank, an iron cap section, an thermal insulation plate, a sprue brick, a connecting bolt, a sealing rubber ring, a reduction gas inlet, a furnace cover, a casting passage, a valve rod, a valve body, a valve plate, an asbestos gasket, a slide gate nozzle and a steel ladle. By the method and device, large and super-large cast steel ingots which cannot be produced by common casting can be produced. The steel ingots produced by the method and device are low in production cost, low in internal segregation, good in internal quality, high in yield, and the like.