In view of the high requirements for sensor sensitivity in detecting marine extremely low frequency electric fields, a new electric field sensor structure design scheme is proposed. Based on the amplification effect of marine electric fields, this study conducts simulation research on sensor structure design, and improves the amplitude of the measured electric field and the sensitivity of the electric field sensor by introducing topology optimization and shape optimization methods. The simulation results show that the sensitivity of the marine electric field sensor after structural design is 2.28 times higher than that of the sensor without a shell.