Composite insulators have been widely used in China’s power grids. Both ends of the insulators operating under high humidity conditions are prone to corona discharge under strong electric field conditions, which will degrade the silicone rubber material of insulator jackets, and cause the insulator to flashover and threaten the safe operation of the power grid in severe cases. In this paper, a composite rubber insulator is used as a test piece, and a multi-needle-plate electrode is used to perform a 100 h corona aging test in a high humidity (RH>95%) environment, and static contact angle, scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) are used to analyze the corona aging characteristics of the samples. The results show that the hydrophobicity of the silicone rubber material is greatly reduced under high humidity conditions, the static contact angle is reduced from 108.45° to 55.47°, and the hydrophobicity is seriously damaged. Cracks appear on the surface of the material, cracks, holes and gullies appear on the microscopic surface. The transverse relaxation time T2 of the material decreases after aging, and the chemical bonds and groups of the internal H atoms change. Some of the molecular main chains break and the microstructure changes, and the hydrophilic —OH is formed, and the electrical properties are reduced. The research results in this paper can provide a certain reference for the design, operation and maintenance of composite insulators in southwestern China or coastal, rainy and foggy areas with high humidity.