Due to the construction of ultra-high voltage transmission lines, the lines will inevitably pass through residential areas. The power frequency magnetic field created by transmission lines will affect the location and design of transmission lines, as well as people’s health. In this paper, the intelligent optimization algorithm is used to optimize the Simulation Current Method (SCM) and study the magnetic field distribution under the transmission line. Firstly, the actual three-dimensional shape of the transmission line is considered in this optimized simulation current method (OSCM). Secondly, we combined the magnetic position error with the magnetic field error to construct the fitness function, and we used the algorithm to optimize position and number of simulated currents. Finally, the finite element method (FEM) is used to verify the calculation results. We calculate a 500 kV 3-D transmission line model and verify that the optimized SCM can reduce the calculation effort and ensure the accuracy of the calculation. In addition, the OSCM solves the problem that the number and location of the simulated current can only be determined empirically. On the one hand, the actual physical shape of the transmission line is considered, which makes the calculation precision higher; on the other hand, the calculation time is much lower than that of FEM. At the same time, it provides a theoretical basis for the magnetic field distribution of transmission lines and has a theoretical guiding significance for the construction of high-voltage transmission lines.