High-frequency transformers, the core component of power electronic transformers, operates under high-frequency, square voltage. At the rising/falling edge of the squared voltage wave, overvoltage and uneven electric field distribution occur among transformer windings, especially among the fast few turns. This can cause uneven electric stress in the dielectric space and local concentration of the electric field in the insulation of the transformer, which can easily lead to premature aging of the insulation or even insulation breakdown. This paper proposes to use COMSOL and SIMULINK co-simulation to calculate the transient electric field distribution of the transformer during the rising/falling edge of the input voltage, providing a basis guideline for the insulation design of high-frequency transformers.