Recently, unmanned-aerial-vehicle (UAV) patrol has become one of the main means of daily operation and maintenance of overhead transmission line. At present, the distance between UAV and power line is mainly judged by the naked eyes, which is a great safety hazard. A method combining the effective electric field value and the effective electric field value with the height is proposed to determine the safe distance range of UAV. The shielding problem and data calibration of electric field measurement are studied, and the electric field distortion coefficient and UAV flight error are solved by gradient descent algorithm. A simulation model of 1 000 kV UHV overhead line is established based on finite element algorithm and Maxwell software, and on this basis, combined with the actual line to test and verify. This method can provide important reference for the distance control, course planning, height adjustment and electric field measurement of power transmission line in UAV patrol.