In order to solve the problem of arrester on-line monitoring device can not accurately obtain the voltage in form of complex wiring, a voltage extraction method is proposed based on frequency and phase difference deviation mean of current and bus voltage. By calculating, analysis and filtering frequency and mean of phase difference deviation of current and voltage, and using the threshold diagnostic algorithm to achieve the actual withstand voltage extraction of arrester. Practical verification demonstrated the effectiveness of the algorithm that the algorithm can reliably distinguish different frequency voltage signal, effectively eliminate interference and accurately extract the arrester actual withstand voltage.