The component contents of silicone rubber composite insulators play a very important role in their quality and service life, but the detecting method of component contents is still inadequate for the electric power department. In this paper, TG-DSC technology is employed to study the thermal decomposition behavior of silicone rubber and its main components (aluminum hydroxide, organic silicone compounds, inorganic materials such as silica), and a method is developed to calculate the component contents according to the thermal weight loss rate in a certain temperature range. The method has been applied to the quality control, operation state evaluation and aging invalidation analysis for composite insulators. A series of composite insulators from several companies are evaluated by this method, and the results indicate that the contents of organic silicone compounds are about 35.9%~40.7%. Layer by layer sections analyses are conducted to running composite insulator skirts, and it is found that the polydimethylsiloxane (PDMS) contents vary with the depth, which reflects the aging state of the housing surface. In addition, this method also provides the component contents information at a selective silicone rubber housing region around fault locations, which can be used for failure analysis for the broken composite insulator. So TG-DSC method for silicone rubber component contents analysis is helpful for the operation and maintenance management of composite insulators, and also is helpful for ensuring the safety and stability of electric power network.