Abstract: The windings of low voltage oil-immersed transformers are often wound by enamelled round wires. In the process of the accurate calculation of temperature rise and hot spot, the influences of the gap between enamelled round wires on oil flow and temperature rise need to be taken into consideration, while the workload of fine modeling and numerical calculation is very large. Aiming at this problem, we propose the idea of applying porous medium theory to the simplified modeling of oil-immersed transformers. Firstly, the validity of porous medium theory applied to the hotspot calculation of oil-immersed transformers is verified. Secondly, based on the porous medium theory, a simplified model of 35 kV oil-immersed transformer winding is established, and the distribution of its temperature rise hotspots is calculated with the Fluent software. Finally, the simulation results of the simplified porous medium model of the transformer are compared with the optical fiber measurement results. The results show that the error between the hot spot temperature of the simulation results and the experimental results is within 4 ℃, and the hot spot position is consistent with the experimental results, which verifies the accuracy of the calculation method of the simplified model of porous media theory. The application of porous medium theory provides a new calculation idea for the 3D fluid thermal coupling of large-scale transformers, which is helpful to simplify the calculation of transformer hotspot temperature in engineering practice.