The invention relates to a transformer oil heating device and method for a large fire test. The transformer oil heating device comprises a transformer oil tank, an oil supply pipeline, a fire separation wall, a return oil pipeline, an electric valve, a manual valve, a heating tank, an electric control system, an hot oil pump, and an electric heating tube, wherein the transformer oil tank is filledwith transformer oil and has an open top or a combustion port on the top; the fire separation wall is arranged between the transformer oil tank and the heating tank to prevent the heating tank from burning and exploding when the transformer fuel tank is burned and to adjust the oil supply-oil return circulation balance. After the transformer oil is electrically heated to the predetermined temperature in steps and in stages, the heating system is stopped and the transformer oil tank has a predetermined oil level. The transformer oil heating device has high heating temperature, high speed, excellent safety and reliability, good repeatability, provides powerful technical support for a large oil-immersed transformer fire test, can be used for the study of fire characteristics of large oil-immersed transformers, as well as for the evaluation of large oil-immersed transformer fire-fighting technology.