In response to the issue of decreased positioning accuracy of unmanned aerial vehicles(UAV) during power line inspection under global navigation satellite system(GNSS) denied conditions, a high-precision autonomous positioning method based on the fusion of power line electromagnetic fields and an inertial measurement unit was proposed. By establishing an electromagnetic field model for transmission lines and designing an inversion algorithm constrained by dual field sources, combined with an extended Kalman filter, the electromagnetic inversion position information was fused with the estimates from the inertial navigation system to achieve high-precision positioning. Simulation results demonstrate that under conditions of conductor horizontal deflection and sag variation, the horizontal positioning mean square error of the method proposed is 0.34 m, and the elevation error is 0.12 m. The proposed method breaks through the dimensional limitations of single field source positioning and achieves centimeter decimeter level positioning of unmanned aerial vehicles in GNSS denied environments, with strong engineering application value.