On-line monitoring plays a more and more important role in maintaining the safe and reliable operation of power grid. In order to supply power to on-line monitoring equipment in a convenient and efficient way, this paper presents a power tapping method to collect power from ground wires using the magnetic induction effect of transmission lines. Cases show that this power tapping method can satisfy the power demand of on-line monitoring equipment. Also, factors such as line current, length of span, grounding resistance and spatial position relationship between transmission lines and ground wires may affect the power tapping efficiency. This paper discusses on the groundwire magnetic induction power tapping scheme, which lays the foundation for further optimization and practical use of power tapping from the ground wire.