The invention provides a device and method for casting a large-sized compound steel ingot by using a graphite electrode ring for preheating electroslag. The device comprises a T-shaped electric conduction crystallizer, supporting arms and a lined electroslag furnace, wherein two transverse arms on a support are respectively used for lifting and dropping the graphite electrode ring and a self-consuming electrode ring; the lined electroslag furnace is used for slagging and pouring premelting slag into the T-shaped electric conduction crystallizer; and an annular bottom water tank is arranged onan ingot introducing plate. The method comprises the following steps of: (1) putting a core rod in the T-shaped electric conduction crystallizer, slagging the premelting slag in the lined electroslagfurnace, and forming a slag tank by pouring; (2) lowering the graphite electrode ring, and causing the graphite electrode ring, the slag tank, the upper crystallizer, a casting power source and the traverse arms to form an electric conduction loop, and heating the surface of the core rod and the slag tank by electrifying; and (3) when a preset temperature reaches, moving away the graphite electrode ring, inserting the self-consuming electrode ring into the slag tank, and carrying out electroslag casting. The method has high utilization rate of heat, can prevent interlayer slag from being generated on the compound interface of the steel ingot, facilitates the improvement of the quality of the steel ingot and simplifies the production equipment.