In order to ensure the reliability and safety of the power supply of the power system, it is necessary to monitor the status of the overhead lines. Overhead line online monitoring devices make up for the shortcomings of traditional offline testing, but require a separate power supply to maintain operation. Most of the batteries in the market are used to supply power to the online monitoring device. However, due to the limited capacity of the battery and short running time, the maintenance cost of the device is greatly increased. Therefore, the online monitoring device can be powered by collecting energy such as the magnetic field around the overhead line. In this paper, a magnetic induction power collection device that can provide 3.3V voltage for an online monitoring device installed on a 10kV overhead line is designed. In order to obtain the maximum power as possible from the overhead line and stabilize the output voltage of 3.3V, this paper designs the toroidal core power coil and the control circuit of the magnetic induction power device. Finally, the experimental test verifies the good performance of the device.