Automatic piloting of unmanned aerial vehicles(UAVs) is vital for the safety of UAVs and the optical image quality acquired by them. In previous effort, the automatic pilot method is based on the image or the GPS (global position system). Inspired by the bird migration using the terrestrial magnetic field, a novel autopilot method based on the magnetic field sensing above the 10 kV OHLs (overhead lines) is proposed in this paper to achieve the routine inspection. The resultant magnetic field above the OHLs is derived from Ampere circuital theorem and the automatic pilot methodology is formulated based on PI (Proportion Integral) control. The method is verified by simulation and the result shows that the inspection track of the proposed method is within the ideal position range for photograph.