When the machine starts work, contaminated transformer oil enters into the primary filter where large size particles will be filtered off; after heating, the transformer oil enters into vacuum separator, under the conditions of high temperature, high vacuum degree, large area and high pumping speed, the water in transformer oil is evaporated rapidly and pumped out as vapor by vacuum system. This vapor will be cooled into water for discharging. The gas in transformer oil is exhausted into air by vacuum pump. The transformer oil is treated again for removing tiny impurity. Then, the clean transformer oil is obtained.