Abstract: Due to the high salt content, high humidity and strong sunshine radiation environment, the aging state of island composite insulators has attracted more attention. It is very important to evaluate the aging characteristics of composite insulator sheds for ascertaining the aging state of silicone rubber. In this paper, 39 samples with different voltage levels, service life and manufacturers were selected. The mechanical properties of insulator sheds were measured by electronic universal testing machine, and the aging characteristics and change of insulator sheds were analyzed. The results show that the tensile strength and breaking elongation of sheds at different parts of insulator are relatively consistent, which does not affect the judgment to insulator properties. There is an obvious boundary between the mechanical properties of broken and unbroken insulators, which is called the broken threshold. The tensile strength threshold of broken insulator sheds is 4.4 MPa, and the breaking elongation threshold of broken insulator sheds is 1.1. Breakage occurs when the mechanical properties of sheds are lower than the broken threshold. With the prolongation of service life, the proportion of broken insulators will increase. In the aging process of composite insulator, with the decrease of breaking elongation, the tensile strength negligibly changes at first, then presents a linear decline trend with breaking elongation, and insulator sheds are likely to be broken in the descending stage.