Recently, several transformer failures happened due to the corrosive sulfur in insulation oil. Corrosive sulfur in oil can attack insulation winding, leading to the decrease of oil-paper insulation characteristic, the decrease of which brings potential safety hazard to the operation of transformer. Adding the metal passivator into the insulating oil is widely used to suppress the formation of copper sulfide in oil-immersed transformer, which can delay the combination between corrosive sulfur and copper ions. This paper deeply studies the suppression effect of metal passivator benzene (BTA) on the formation of copper sulfide, and the effect of BTA on the properties of insulating oil. The results show that high concentrations of benzotriazole (BTA) showed a good anti-corrosion effect when the oil contained different amounts of the metal deactivator. However, the content of copper ions dissolved in the oil would increase on the contrary. Therefore, the content of BTA added into the insulation oil should be controlled.