The power electronic devices of ITER will bear a magnetic field strength of more than 5 mT, which may affect the operation of the devices and cause different degrees of damage to the devices. Therefore, these power electronic devices need a magnetic field anti-interference test. The core of the test facility is a magnetic field coil. First, the mathematical models of two-coil and multicoil systems considering the cross section are established. The general formula, optimization design objective, and optimization design process of the coil parameters are obtained, and the optimization scheme of the optimal solution is determined. Then, the multicoil systems are analyzed according to the actual design requirements and the actual site conditions. Some parameters that play a major role are used in the magnetic field distribution as variables to analyze and select the appropriate coil structure. Finally, the four-coil-group system with equal side lengths is applied as the coil structure based on the design requirements, power loss, weight, and manufacturing difficulty. Finite element software simulation results and experimental results prove the feasibility and correctness of the theoretical analysis.