The silicone rubber of composite insulator shed and housing is a kind of polymer organic material. Due to the effect of environmental stress under the longterm operating, aging phenomena such as the color fading, chalking, cracking, brittleness and decline of hydrophobicity will appear on it inevitably. The performance of insulators affects the safe operation of transmission lines directly. The aging of silicone rubber is related to the complex operating environment and the aging resistance ability, and it is represented as the change of key components directly. Therefore, the sampling inspection for composite insulators should be carried out by operating maintenance departments. In this paper, the key components of silicone rubber analysis schemes are proposed and the rubber content is defined. To measure the components of silicone rubber, 13 kinds of flat silicone rubber samples with certain content of siloxane and different content of aluminum hydroxide and white carbon black are prepared. Methods of density and thermos gravimetric analysis are adopted, then the results are fitted by the linear regression. Silicone rubber samples of the shed inner layer from different manufacturers are selected to verify the consistency of the results obtained by the two methods. It is shown that the rubber content of silicone rubber can be calculated by either the density or the remaining mass ratios at the critical turning point of thermal gravimetric (TG) curve. The field or offline analysis can be carried out according to the actual situation. The operating status of composite insulators can be assessed by the proposed schemes conveniently and reliably, then safe operation of transmission line can be promoted.