The present invention is directed to a special cleaning fire-resistant oil and preparation method thereof, which comprises the components of base oil trixylenyl phosphate, tricresyl phosphate and di- (tert-butyl phenyl) phenyl phosphate at weight ratio of (0.5-0.8):(1.2-1.5):(1.1-1.13), 0.9 wt% of epoxy soybean oil isooctyl phenyl, 0.4wt% of benzotriazole and 0.6wt%-1.5 wt% of EDTA. The medium and high voltage resistant oil provided by the present invention has characteristics of low chlorine content and acid value, high ignition point and boiling point, and advantages of good lubricity, abrasion resistance, aging resistance, anti-foam, anti-flammability and thermal stability etc. The conditioning system used in large generating units of more than 300MW is a reliable working system to guarantee the safe operation of generating units, and the products not only can be applied in high-temperature, high-pressure and hydraulic lubrication systems, but also can be extended to other systems of high-temperature heat source or near the open flame.