Transformer oil is one of the basic materials used in power transmission system, and mainly for insulation, cooling and heat dispersion of oil-filled electrical equipment, such as transformer, reactor, mutual inductor and switch, etc. A main function of transformer oil is to provide good electrical insulation performance. Through strict linkage control in manufacturing,transporting and using of transformer oil, the oil provides extremely low conductivity and high breakdown voltage, thus reducing power loss of electrical equipment. Electrical equipment will generate plenty of heat during power transmission and transformation process, so the transformer oil is required to provide good heat-transmission capability. Through hot and cold convection and circulating reflux, transformer oil with proper viscosity and low pour point can send heat out in time to avoid over temperature of electrical equipment like transformer. In addition, transformer is mainly composed of copper wire, iron core and non-conductive cellulose etc. To ensure a long service life of transformer, transformer oil must provide good antioxidant capacity.