After the SOD was treated with different strength of electric field, the interactional mechanism of electric field on SOD activity was studied by circular dichroism. The activity of SOD was enhanced under the treatment by different strength of electric field. Circular dichroism spectra showed that the secondary structure of SOD was greatly changed by electric field, as β-sheet and β-turn contents decreased, while α-helix and random coil contents increased. It was considered that the increase of the α-helix structure near the active center would lead to the inactivation of SOD. The result of this study has important meaning to explain the biological effect of electric treatment seeds.