Abstract: Considering the finite conductivity of the earth，the horizontal electric field of the near earth is more significant influence on the power cord and electronic equipment. Therefore，the earth conductor is supposed to be a noble ideal，in this case，the transmission line equation of the lightning electromagnetic field is derived，which needs to be revised. We use the modified transmission line equation and the various parameters of the earth and get the spread of lightning overvoltage on the transmission line. The impact on the limited earth conductivity is discussed and analyzed.

Key words: lightning electromagnetic field, finite conductivity of the earth, transmission line equation