For their outstanding resistance to the contamination flashover, composite insulators have been widely used in the EHV/UHV transmission lines. In this paper, an abnormal heating defect in 500 kV AC composite insulator operated in Shenzhen has been studied. With the visual test, AC voltage withstanding test, water diffusion test, absorption test and FTIR test, it is confirmed that water absorbed in the sheath and polar group generated from the aging process are the main reasons of the heating of composite insulators, and the heating process will accelerate the aging process at the same time. The test results also show that during the performance evaluation of newly accepted composite insulators, the saturation absorption water of silicone rubber should be considered besides the interfacial resistance to hydrolysis performance.