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Abstract Composite insulators are widely used in the power transmission system. Operating in the outdoor environment, the composite insulator will be inevitably polluted by natural contamination with the increase of service life, and contamination on the surface is one of the main reasons for the abnormal temperature rise of the insulator. In this study, the rated voltage was applied to the kieselguhr sample under a high‐humidity environment, and its temperature rise characteristics and leakage current were tested. The research results show that the temperature rise in contamination falling‐off region is significant and the leakage current is greater than the clean sample. By analysing the phenomenon with the help of the circuit model, it is concluded that the high resistance and voltage in contamination falling‐off region leads to high heating power of the polarisation loss.