In order to discuss the three-dimensional distribution characteristics of electric fields near transmission lines based on the FEM, a three-dimensional calculation model and the sag equation of transmission lines is established. The 3D electric field distribution of extra-high voltage transmission lines of 500 kV named Dongsi Line is calculated by 3D calculation model. Then the results of comparison between FEM and charge simulating method verify effectiveness of the 3D calculation method. Furthermore, the influence of the factors such as the arrangement mode of phase traverse, the size of split traverse and topographic relief on the spatial electric field strength and distribution characteristics are calculated and analyzed. It’s the result of comparison between the calculation values and the measurement values that the variation trend of electric field intensity was similar and the distribution features were consistent. The results show that the deviation from the actual value is greatly reduced 60.94 % to 9.58 % which compared with the calculation results of the two-dimensional model. The calculation method in this paper provides an important theoretical basis for the calculation of three-dimensional electric field near the transmission line based on FEM and is of great significance for the calculation of power-frequency electric field and the location of transmission line.