In order to provide a stable and convenient power supply, based on the principle of electromagnetic induction, a wireless capacitance power take-off device is designed, which can maximize the use of space energy. It is important for the monitoring of the operation status of transmission line. First of all, the structure of capacitance power draw-out is introduced, and the calculation relationship between the output power of energy extraction device and the high-voltage stray capacitance, low-voltage stray capacitance, load resistance and other parameters is deduced. It is found that increasing the area of the sensing electrode, reducing the capacitance value of sensing electrode and increasing the load resistance value reasonably can effectively increase the output power of energy extraction device. Secondly, specify software is used to simulate the influencing factors of capacitance power supply. Finally, a high-voltage experimental device is designed with Matlab software, and the actual situation is simulated and tested. The experimental results show that 1 MΩ load can get 0.12 mW power when the power supply voltage is 10 kV and the coupling capacitance is 14 pF.