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composite insulator has been widely used in highvoltage electric power transmission system owing to its advantages such as light weight, anti-pollution flashover and less routine maintenance. With the increasing number of compositor insulators and their running years, the aging of compositors draws attention to the related industries. Considering the aging problems of composite insulator, the material performance of a silicone rubber composite insulator which has worked for more than 10 years on a 500KV transmission line at Guangzhou in China has been analyzed. Judged from the surface, a non-uniform aging characteristic on the different surfaces of the insulator sheds and sheathes exists. The contact angle measurements made on surfaces from different positions confirm the visual aspect of non-uniform aging. In order to explain the phenomenon, the electric field of different positions is computed through finite elements methods. Then the shed and sheath samples on the insulator's various positions were analyzed with Scanning Electron Microscope (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). At the same time, Thermally Stimulated Current (TSC) was used to find the inner charge difference of both sides of sheds from differentpositions. It is found that electric field intensities and ultraviolet of sunlight are the main factors influencing the aging of the silicone rubber composite insulators. The results will be helpful to the researches of aging problems from composite insulator and its antiaging optimization design.