In China high temperature vulcanized (HTV) silicone rubber has been applied to manufacture the sheds and sheath of composite insulators for decades. Being exposed to environmental stressors including sunlight, UV radiation, oxidization, electrical stress and moisture throughout the whole operation period, various aging phenomena could be observed, among which color fading and hardening are two important indexes which may be directly related to the performance of composite insulators. According to polymer aging theory, thermo-oxidative could be the main aging reaction leading to the hardening of HTV silicone rubber and in this paper, Thermogravimetric Analysis (TGA) combined with Differential Scanning Calorimetry (DSC), Infrared Spectroscopy (IR) and Mass Spectroscopy (MS) are applied to study the degradation and decomposition process of HTV silicone rubber in air (oxidizing atmosphere) and nitrogen (inert atmosphere) respectively. 18 composite insulator specimens have been collected and their color fading degree, hardness, crosslink density as well as thermal performance have been measured and calculated. A correlation analysis showed that physical crosslink density had a strong positive relation with hardness of specimens whereas chemical crosslink density showed negative relations with hardness. It is concluded that TGA is an effective method to simulate and study the thermo-oxidative aging process of HTV silicone rubber and the measurement results of crosslink density shows that decomposition and over crosslinking reactions have taken place simultaneously in HTV silicone rubber and led to the aging of composite insulators together.