The invention relates to a device for replacing an interface flange sealing ring of a main transformer with oil. The device comprises a main transformer body, an oil conservator, a vacuumizing three-way valve, a dynamic vacuum system, transformer oil arranged in the main transformer body, a main connecting pipe arranged on the lower portion of the main transformer body and an oil drain valve, wherein the oil conservator is connected to the top of the main transformer body through a communication valve and a gas relay; and the vacuumizing three-way valve is arranged at the top of the main transformer body and is connected with the dynamic vacuum system. The device further comprises an oil level sensor, wherein the oil level sensor comprises an extension rod with electric contacts at two ends, and the electric contacts at the two ends of the extension rod are respectively an initial end electric contact and a tail end electric contact. According to the invention, when a main transformerconnecting pipe and a valve are damaged, leak and need to be treated or a sealing element needs to be replaced, oil flow at an opened pipe orifice or a connector can retract and cannot flow out in a short time, thereby ensuring the work of workers to be smoothly completed, and also being capable of avoiding the risk that external air is sucked into the main transformer due to excessive vacuum pumping.