The on-line monitoring device of transmission line in Xinjiang Province cannot work normally in the extremely cold environment, which brings some difficulties to the operation and maintenance of transmission line. In this paper, the implementation scheme of power tapping from OPGW was put forward. Based on EMTP simulation platform, 500 kV ac transmission line model was established, which simulated the induced voltage and current, and the dual insulation mode is proposed. The results prove that the induced voltage and current of ground wire are positively correlated with the spacing and load power, and are basically unaffected by the soil resistivity; when the span is 300 m and the load power is 500 MW, the output power of the ground wire energy-collecting device is 137.8 W, fully meeting the power supply demand of on-line monitoring device in the transmission line; under the condition of long-term extremely cold temperature -45 ℃, by using nano porous silica insulation membrane and infrared radiation heating, it still can ensure ideal temperature conditions,which is not less than -30 ℃; after the insulation measures are adopted for the power supply, the mean rate of no failure in online monitoring device is increased from 85.9 to 99.6%, and the reliability is significantly increased. The research results provide a design idea of stable power supply for on-line monitoring devices in extremely cold regions.