Abstract: The principle, structure and property of a kind of ground-based double-boom electric field detector is described here. The atmospheric electric condition represented by the typical recording is analysed. The conception of equipatition of frequency is proposed. The characteristic resistance value is calculated through the analysis of the typical detected data. According to the analysis of the joint measurements with an electric field mill, a method is proposed to distinguish the fluctuation of the local electric condition and the influence from the upper atmospheric electric condition.