Abstract: As a key component of transmission lines, the insulation performance of composite insulators directly affects the reliability and security of power supply. By investigating the composite insulator shed samples of different operating years, the hydrophobicity, infrared spectrum, DC high field conductance and isothermal relaxation current test were carried out to study the aging characteristics. The results show that both the surface hydrophobicity of the sample and the content of the side chain of the silicone rubber molecular chain decrease with the increase of the operating years. In the range of 0.4-40 kV/mm field intensity, the J-E curve is two-stage, and the threshold electric field decreases from 6 kV/mm to 3.3 kV/mm. The initial value of isothermal relaxation depolarization current is positively correlated with the aging degree, and the trap depth difference is small but the trap density increases significantly. The volume resistivity of the samples with the greatest aging degree is the lowest, and the threshold field strength is the lowest when the samples fall below the standard line of volume resistivity. The results of physical and chemical properties are consistent with the threshold electric field, the initial value of depolarization current, and trap density, which can be used as the parameters for aging characteristics.