Electrical bushings, as the key insulating medium and physical fixing components of power transformers, play an important role in the reliability of power systems. However, the increase in transformer operating voltage causes the bushing to be subjected to higher electric field stress, which further leads to partial discharges or even electrical breakdown of the insulating medium. In this study, we simulated the electric field distribution of the high-voltage transformer bushings based on the finite element method. The results show that dielectric functionally graded materials used as insulation medium help tune the electric field distribution and reduce the maximum electric field stress (Emax) of high-voltage bushings. Compared with the uniform insulation (permittivity of 4), a 4-layered graded insulation (permittivity of 4, 5, 6, and 20) reduces Emax from 25.2 kV/mm to 13.9 kV/mm at an operating voltage of 220 kV. Furthermore, we have fabricated an electric field-assisted graded composite that can be used as a potentially superior insulating material for high-voltage bushings. These results indicate that the electric field optimization strategy based on dielectric functionally graded materials is a promising method for suppressing the maximum electric field stress of high-voltage bushings.