In this paper a new method is described in which an annular metal sheet coated with silicone rubber is added at the base of the conventional insul ator cap to improve the insulator unit＇s performance under both clean and pollut ed conditions. The annular metal sheet is connected to the insulator iron cap an d makes the electric field distribution more uniform, which increases the corona onset voltage. The annular metal sheet also makes the electric field direction on the insulator upper surface approximately perpendicular to the insulator surf ace, rather than parallel, as on the conventional insulator, therefore the devel opment of the discharge is restrained. At same time, the annular coated with sil icon rubber which possesses good pollution-proof performance can increase the c r eepage distance, reduce leakage current and raise the flashover voltage. The exp erimental results obtained coincide with the results of theoretical analyses. Th e fine performance of novel insulator is approved. After fully immersing the nov el insulator into the sodium chloride solution at density of 10g/l, its flashov er voltage is multiplied, the corona onset voltage increased more than 70% and t he leakage current decreased nearly 30% under the same polluted conditions.