Insulation margins are critical to the safety and reliability of transformer operation. However, with the increase of operation years, aging of insulation materials, accumulation of moisture, and impurities occur inside the transformer, which leads to the degradation of insulation performance, and then leads to the change of insulation margin. In this paper, a dynamic insulation margin assessment method for transformers, which takes into account the temperature-moisture coupling, is investigated. By combining the multi-physical field distribution and material property change relationship, the dynamic insulation margin results of transformers under different years are obtained. The results show that, with the increase of operation years, the insulation margin of the same measurement point in the 48th year is about 10.95% lower than that in the 1st year.