Abstract Copper, as one of the most hazardous metals to transformer insulation systems, has attracted a great deal of interest from scholars at home and abroad. Unfortunately, the specific type of copper that causes catalytic aging of oil-paper insulation is still controversial. Therefore, in this paper, the effect of the electronic state evolution of copper in different oxidation states on the aging performance of the insulation systems was investigated by characterization techniques and performance evaluation. Cu2O has a stronger catalytic oxidizing effect on insulation system than CuO. This study elucidates the electronic state evolution of different oxidation states of copper and its influence on the insulation performance, which lays a good foundation for the advancement of transformer fault warning technology.