At present, the lack of an efficient and stable energy harvesting technique for high-voltage direct current (HVDC) transmission lines hinders the deployment of self-powered sensors. Due to the limited switching frequency of electronic devices, the DC current in HVDC lines contains AC harmonic components, which serve as an energy source for magnetic field energy harvesters. This work proposes a wideband harmonic magnetic field energy harvesting method for HVDC lines. The designed harmonic magnetic field energy harvester (HMFEH) consists of a non-closed core and coil, avoiding core saturation caused by strong DC magnetic fields while enabling efficient conversion of harmonic magnetic field energy to electricity based on electromagnetic induction. Additionally, a second-order bandpass resonant network (SOBRN) is developed to achieve high energy transfer efficiency across multiple frequency points. Theoretical analysis and experiments confirm the effectiveness of the proposed system. Results show that the SOBRN increases the maximum load power by approximately 13.3 times compared to the uncompensated system and by 1.82 times compared to the traditional single-stage resonant network.