In order to realize resource sharing, 5G communication equipment install on transmission tower, and the influence of electric field on the position of communication equipment should be considered. For the electric field calculation at the place around the surface of the transmission tower, the direct use of the charge simulation method (CSM)will have a large error. The finite element method (FEM) can fully consider the influence of the complex structure on the electric field. However, the electric field of transmission lines is open field, and the size difference between the conductor and tower is large, the finite elements mesh division is difficult, the calculation is large and the efficiency is low. In this case, the open field is cut into a small rectangular region by combining CSM and FEM. The potential distribution on the boundary of the cuboid is calculated by the CSM. Then the potential on the boundary interface is applied to COMSOL, and the surface electric field of the tower is calculated by the FEM. The calculation method is verified by the practical case of shared transmission tower. The method can reduce the amount of FEM simulation calculation on the premise of ensuring the accuracy of calculation, and it is suitable for the calculation of electric field on the surface of each voltage level transmission tower.