Recently, unmanned-aerial-vehicle (UAV) patrol has become one of the main means of overhead transmission line daily operation and maintenance. At present, the distance between UAV and power line is mainly judged by the naked eye, which is a great safety hazard. In this paper, a simulation model of 1000kV UHV overhead line was established based on finite element algorithm and Maxwell software, and a method combining the effective electric field value and the effective electric field value with the height was proposed to determine the safe distance range of UAV. At the same time, the shielding problem and calibration of electric field measurement are studied, and the electric field distortion coefficient and UAV flight error are solved by gradient descent algorithm. 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.