The bar and the cloud microcosmic appearance character of the electric erosion is described in this paper.The electric erosion mechanism of the gear surface has been set forth by setting up the model of electric erosion of the discharge not contacted and the model of electric erosion of discharge contacted. The electric erosion of the discharge not contacted would be taken place when the voltage is stronger and the lubricating oil film formed better between the space of the gear surface. The electric erosion of the discharge contacted would be taken place when the voltage is lower and the lubricating oil film formed not better between the space of the gear surface. The failure of the gear is accelerated by the action of the electric erosion and the friction wear together.