Abstract: A approximate method for energy gap calculation of oxides is introduced in detail, and is applied for the gap culculation of high-T_C copper oxide superconductors at normal state. It is revealed from the result that, most oxide superconductors at nomal state have the gap values within 8.50-8.80eV. In the same system, the variation of Cu-O layer structure results in the increase of T_C upon the decrease of E_g, while in YBa₂Cu₃O_y, the variation of oxygen content results in the increase of T_C upon the increase of E_g. Further analysis shows, the E_g values of high-T_C oxides at nomal state have a close relationship with superconductivity, and the average ionization potential is the major factor in determining the E_g values.