Composite insulators are composed of a core and a coat with metal attachments. The core is made of glass fiber reinforced plastics impregnated with an epoxy resin. This material is a kind of high strength and non-brittle composite. But the interface in an adhesive layer between a mandrel and an umbrella is the weak link, which is susceptible to mechanical damage in the areas with significant changing temperatures. The Instron1186 electronic universal testing machine is adopted to carry out the material mechanical property tests through controlling displacements, and the tensile properties of composite insulators under high and low temperatures are studied. The test shows that the mechanical failure load of composite insulators depends on the connection strength of the metal tool and the mandrel, and the main features of the failure are the slip of the end of the metal tool. Because the thermal expansion coefficient of the end fittings is significantly different from that of glass fiber reinforced epoxy mandrel, the mechanical properties of composite insulators are lower than normal temperature at high temperatures or low temperatures. Especially at a high temperature, the bonding-slip property of the metal tool and the core rod is greatly reduced.