Based on electrical and chemical analysis in oils of some fault transformers in Shenzhen, it indicates that the corrosive sulfur contained in insulating oils may cause faults. Corrosive sulfur would react to copper wire and engender Cu2S during the operation of transformer. Because of the electric characteristic of Cu2S, this substance will penetrate and pollute insulating paper of the wire, and then weaken the insulation intensity. At last it would wear out coil insulation. Experiments indicate that adding passivator to insulating oil is an effective method to solve the problem. Passivator can form a protection layer on the surface of copper, which can avoid copper corrupting and can stop copper deposition in the presence of potentially corrosive sulfur compounds as well.