The coil copper resistance of the large-power rectifier transformer used in the metal smelting, chemical and electrolytic industries increases as the temperature increases, and so does the corresponding basic copper loss. The cooling system of the transformer ensures that the rectifier transformer runs under specified operation conditions of temperature rise. In addition, it reduces the windings' consumption by lowering the temperature. However, the application of cooling system will consume a sum of energy as well. It depends on whether the whole system reaches energetic minimum. This paper made a careful study on the relationship between the power dissipation of the rectifier transformer, the power consumption of the cooling system and temperature according to the operating data of an aluminum company. The research reveals the optimal temperature rise and cooling power for the minimum energy loss of the rectifier transformer, which paves a way for realizing the rectifier transformer's comprehensive energy saving.