In order to control the concentrated distribution of electric field on the surface of three-pillar insulators and inserts, a geometric-dielectric optimization method based on finite element model(FEM) combined with evolutionary algorithm (EA) is proposed. Firstly, geometric and surface dielectric parametric models of gas insulated line(GIL) three-pillar insulators are established. The maximum tangential field strength of insulator surface E_t-max, the non-uniform coefficient of tangential field strength F_t, and the maximum normal field strength of insert surface E_i-max are taken as optimization targets, and geometric-dielectric optimization calculation is carried out by cyclic FEM with evolutionary algorithm. Compared with the results before optimization, E_t-max decreases from 11.3 kV/mm to 9.47 kV/mm, F_t decreases from 1.56 to 1.34, and E_i-max decreases from 31.9 kV/mm to 13.4 kV/mm. The geometric structure of the optimized three-pillar insulator has the characteristics of dumbbell type, the distance along the surface is increased, the overall profile is a circular arc with greater curvature, and the surface dielectric gradient after optimization is distributed in a "scoop curve". The results of optimization design can provide reference for the subsequent manufacture of three-pillar insulators, and the technical route of this paper can also be used for the optimization of other insulation parts.