Abstract: Due to the physical and chemical properties of natural ester oil, the temperature rise of transformer with natural ester oil as cooling medium is higher than that of mineral oil transformer with the same structure. In order to solve the problem that the temperature rise of natural ester oil transformer is high and there is no reliable analytical calculation formula, we take an SWFZ−40000/132 natural ester transformer as a research object to establish a refined simulation model of electromagnetic field and fluid field of transformer. Moreover, we accurately calculate the loss of transformer, analyze the influence of axial oil duct height on the temperature rise of transformer winding, and reasonably design the transformer. The main insulation strength of transformer is checked to ensure the electrical safety of transformer. Finally, the correctness of theoretical analysis is verified by the test of transformer.