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The horizontal electric field of lightning is an important parameter for calculating the lightning induced overvoltage on overhead power lines. In order to obtain an accurate value of this parameter, we use the Finite Difference Time Domain (FDTD) method to calculate the lightning electric field under different ground conductivity as well as different distances from the lightning return stroke channel. The results show that in the case of the uniform ground conductivity, the greater of the ground conductivity and the shorter of the distance, the closer the waveform of the horizontal electric field and the input current, because the component of the radiation field in the electric field is relatively small. By increasing the conductivity of the ground or increasing the distance, the calculation results decrease till bipolar characteristic appeared. For the case of the layered ground conductivity, the computed results are influenced more greatly by the upper soil conductivity, but the equivalent resistance is changed by the underlying ground to a certain extent, so that the calculated results are different with the case of uniform ground conductivity.