Abstract: In order to improve the electric field distribution at the gas-solid interface of the insulator, the concept of the surface functionally graded material (SFGM) is proposed, and the iterative method is used to optimize the surface relative permittivity distribution of the 220 kV GIS basin - type insulator. The surface relative permittivity distribution trend of the optimized SFGM insulator is positively correlated with the electric field distribution along the surface, which gradually decreases from the central conductor to the outer shell. Compared with the conventional insulator, the optimized C-SFGM insulator can significantly reduce the electric field strength on and around the insulator, thus improving the electrical resistance performance of the basin-type insulator.