Composite insulators are increasingly employed in EHV/UHV transmission lines due to their light weight and excellent mechanical properties. However, with prolonged service life, composite insulators are susceptible to hidden defects and heating, leading to serious consequences. In this paper, the multiphysical field finite element model for real?size 500 kV transmission line composite insulators is established, and the distribution of electric field and temperature field under conductive faults at high voltage end, low voltage end and middle position are calculated. The research findings yield the following conclusions: There are substantial changes in the electric field and thermal field of the insulator strings under conductive faults. Specifically, when conductive faults are introduced at the high?voltage end, low?voltage end and middle position, the maximum electric field at the tip of insulator umbrella skirt can reach 17.45, 15.95 and 5.62 kV/cm respectively, and the corresponding temperature rise can reach 9.3, 4.4 and 1.8 ℃ respectively, representing a maximum temperature rise 10.6 times higher than that under normal working conditions. In addition, the temperature distribution along the insulation interface exhibits distinctive characteristics under conductive fault conditions at different positions. For example, in the case of a fault at the middle end, two obvious temperature peaks are observed. Finally, an infrared thermal image test is carried out, and the results are in good agreement with the simulations presented in this paper.