Enclosed transformer oil purifier applies for filtrating and dehydrating transformer oil, mutual conductor oil, switch oil and other kinds of insulation oils. The purifier adopts high effective vacuum and the advanced stereo-evaporation technology, which are also incorporated with a number of state-of-the-art patent technologies. These technologies enable a highly effective removal of water, gas and impurity without switch off of power, shutdown of machine or change of oil which allows transformers running as energized. With these technologies, the oils withstand voltage is increased and the oil quality is improved, which recovers the used oil back to the specifications of new oil and at the same time ensures the safe operation of power generation equipment. The product is widely applied to areas of power station, power company, transformer substation, railway, petroleum, chemical and metallurgy. The product is also a special facility of electric power and industrial enterprises for treatment of various insulation oils including non-conformable transformer oil, mutual inductor oil and circuit breaker oil, as well as a best choice for vacuum oiling of power equipment and drying of moisture of power equipment.