As renewable energy increases, the demand for substation construction also needs to increase simultaneously. Before a substation is built, its environmental magnetic field assessment is particularly important to people’s health, and most of this environmental magnetic field assessment is only for the outside of the substation. In recent years, the safety and health of occupational personnel have been gradually valued, so the assessment of the environmental magnetic field inside the substation is also very important. Finite element analysis (FEM) is currently most commonly used for accurate environmental magnetic field assessment. However, its calculation consumes a lot of time and computer resources. Therefore, this study proposes a simplified model method for the main magnetic field source inside the substation, that is, the power transformer. This method can not only simplify a lot of simulation time, but also ensure that the overall error of the magnetic field simulation value above 2,500 mG is within 5 %.