In the electric power industry, the requirements for capacity, voltage level and compactness of transformers are getting higher and higher, which lead to significant problems, such as increased loss and more serious hot spots phenomena. Based on the three-dimensional finite element method, this paper establishes the bidirectional coupling between electromagnetic field, flow field and temperature field to calculate the temperature rise of 400 kVA oil-immersed transformer at full load and compare it with the results obtained by the heat-flow unidirectional coupling method (constant loss). The results show, due to the interaction between electromagnetic field and temperature field, the coil loss density increases, but the core loss density does not change. The hot spot temperature of coil increases and the location of coil hot spot changes. The temperature and position of the core hot spot do not change. The loss density of coil decreases as the inlet velocity increases, the hot spot temperature of coil drops faster. When the inlet velocity exceeds 0.5 m/s, increasing the inlet velocity can no longer effectively promote heat exchange. With the increase of the inlet velocity, the interaction between electromagnetic field and temperature field is weakened, but it still has a great impact on the loss density of coil. The multi-physics coupling method is of great significance to improve the accuracy of transformer temperature rise calculation.