Abstract: When the oblique cantilever insulator is iced in a low-temperature high-humidity environment, the icicle of small sheath extends to the edge of large sheath and forms a tiny air gap, which will cause a significant decrease in the creepage distance after breakdown, and even cause flashover accidents in serious cases. In this paper, a special-typed sheath structure with diameter perpendicular to the horizontal plane is designed by analyzing the droplets sliding trajectory and the growth process of icicle on the surface of the existing oblique cantilever insulator sheath. Compared with the existing insulator, the circumferential structural parameters of the special-typed sheath is asymmetrical, and the minimum creepage distance decreases. In order to meet the requirements of creepage distance parameters required by the specification, the parameters of the number and the diameter of booster sheathes that need to be installed locally are calculated and determined. The electric field simulations are carried out on the insulators before and after optimization, the insulator models are manufactured by 3D printing technology, and the influences of sheath structures on the electric field intensity distributions and icing characteristics are analyzed. The simulation and test results show that, during the sliding process of the droplets on the optimized special-typed sheath surface, the droplets at the edge of small sheath will not flow through the edge of large sheath, no ice bridging occurs between the icicles of the adjacent sheath edge, and the icing flashover voltage of the optimized insulator is 13.2% higher than that of the original insulator. Compared with the electric field intensity distribution of the three-dimensional transversal near the small sheath icicle, the peak value of the transversal field strength intensity after the sheath optimization significantly decreases, and the peak value decreases by 77.84 %. The special-typed sheath structure of oblique cantilever insulator proposed in this paper can provide reference for improving the icing performance of oblique cantilever insulator of catenary in a low-temperature high-humidity environment.