To address the issue of energy overflow in online energy harvesting devices caused by large fluctuations in current in power transmission lines, a method for stable online energy harvesting on transmission lines based on adaptive adjustment of auxiliary magnetic core impedance is proposed. By adding auxiliary magnetic cores and constructing a multi-magnetic circuit model for the energy harvesting branch and signal branch, the goal of stable energy harvesting is achieved. Based on the relationship between busbar, energy harvesting branch coil, and signal branch coil currents, a corresponding energy harvesting model is built on the Matlab/Simulink simulation and test platform. The operational effects of the energy harvesting branch are investigated at different current levels and with or without auxiliary magnetic cores. The costs, control complexity, and reliability are also analyzed. The simulation and test results demonstrate that even as the current in the power transmission line rises from 30 A to 100 A, the output voltage of the energy harvesting load can still be maintained within a stable range. This confirms that the output voltage of the energy-harvesting load can be maintained without additional power supply modules, meeting the power supply requirements of online monitoring devices.