The insulation material epoxy resin mean molecular weight to its solidification electrical performance influence is discussed. In view of the bisphenol A epoxy resin with different mean molecular weights, Finite Element Method (FEM) are introduced and electric field calculation equations are established to calculate the field distribution of basin insulator and its around region in GIS. Representative potential distribution curves are shown. The tangential and normal electric filed intensity maximum of basin insulator is calculated. The positions of electric field intensity maximum are also given.