To address the issue that current transformer (CT) energy harvesting devices are unable to provide real-time power supply to online monitoring equipment under low current conditions in transmission lines, a resonant CT energy harvesting method and its corresponding optimization method are proposed. First, a comprehensive mathematical optimization model integrating quality factor, mutual inductance, and losses is established through theoretical analysis. Subsequently, the non-dominated sorting genetic algorithm II (NSGA-II) is employed to perform optimal design, determining key parameters such as CT mechanical dimensions, compensation capacitance, and number of turns. This significantly enhances the output power of the CT energy harvesting device. Finally, simulation and experimental results demonstrate that under a transmission line current as low as 15 A, the proposed self-resonant CT energy harvesting method achieves an output power of 2.186 W—approximately 6 times higher than that of conventional CT-based methods under the same conditions—thereby enabling uninterrupted power supply for transmission line online monitoring equipment.