At present, the self-potential method is widely used in the measurement of marine electric fields. Non-polarized electrodes such as Ag/AgCl or super-stable electrodes such as carbon fiber electrodes are immersed in seawater to measure the voltage between the electrodes and indirectly obtain the electric field. On the one hand, the self-potential method requires the electrodes to maintain electrical contact with seawater during measurement. The characteristics of the electrode may change gradually under the influence of seawater, which will inevitably affect the measurement stability. On the other hand, in the self-potential method, since the measurement sensitivity is proportional to the electrode spacing, the electrode spacing is usually several meters to several kilometers. The difference in environmental temperature and salinity between the two electrodes will cause voltage drift, resulting in measurement errors. In response to the above problems, this paper proposes the principle of electrical modulation for measuring marine electric fields. By controlling the electrode connection through electronic switches to modulate the electric field, an alternating electric field is generated to induce an induced current as the electric field sensing signal. When the detection electrode is insulated from seawater and does not conduct, the marine electric field signal is picked up to prevent the electrode from reacting with seawater, and the characteristics of the detection electrode can remain stable for a long time. Moreover, since the intensity of the sensing signal is negatively correlated with the electrode spacing, the electrode spacing is much smaller than that of the self-potential method, which greatly reduces the influence of the potential difference and drift caused by environmental factors on the measurement signal and the resulting measurement error. A simulated underwater electric field test platform was built to experimentally verify the proposed principle, and the direct current electric field in the simulated seawater was measured by applying this principle. The designed electrode spacing was 25 mm and the electrode dimensions were 50 mm×50 mm×1 mm. The experimental results show that the proposed method achieves an electric field measurement sensitivity of 117.87 mV/(V/m), a minimum resolution of 5.786 μV/m, and a drift range within 10 hours of less than 50.9 μV/m.