Abstract: In order to detect the DC electric field strength under extra-high voltage DC transmission lines, a new chopping measurement method of DC electric field strength is presented. The DC voltage signal caused by induction metal pad of electric field sensor is chopped. At first, the DC voltage signal is transformed to a low frequency AC square wave signal when the detecting sensor is no mechanical movement. Then the DC electric field strength is measured by using the operational amplifier circuit with extreme input impedance. The hardware and software of the measuring system are designed. The experiment testing results show that this method is feasible and effective.