In order to obtain a more accurate three-dimensional distribution of the magnetic field under the transmission line, an intelligent optimization algorithm is used to optimize the number and position parameters of the simulated current in simulation current method, which solves the problem that the position and number of the simulated current in the normal simulation current method can only be determined by experience, so as to improve the calculation accuracy. A three-dimensional calculation model based on the actual physical shape of transmission lines was established to calculate the three-dimensional magnetic field distribution of Dongjing-Sijing 500 kV line in Songjiang district, Shanghai by different methods. By comparing the calculation results with the actual measurement results, the results show that the calculation error of the optimized simulation current method is 4. 54%, which is 7. 67% lower than the 12. 21% of the traditional calculation method. It provides a theoretical basis for optimized simulation current method to calculate the magnetic field distribution of transmission lines, and has a theoretical guiding significance for the construction of high-voltage transmission lines.