Studying the oil-natural-air-natural (ONAN) transformer’s temperature field distribution characteristics and hot spot temperature rise during operation is a key step to evaluate the thermal insulation life of this type of transformer. Firstly, considering the transverse oil passages between the windings and the guiding effect of the oil baffle on the oil flow, this paper takes 35 kV ONAN transformer windings as the research object, which establishes the corresponding electromagnetic-thermal-fluid coupling model and uses the finite element method to calculate the overall distribution of the internal winding temperature field. The calculation results show that when the ONAN transformer reaches thermal equilibrium, the axial temperature distribution of the windings is extremely uneven due to the influence of the transformer oil flow rate, whose difference between both ends is as high as 17.4℃. However, the existence of the oil baffle increases the flow velocity of the inter-turn transverse oil passage to 12 times of the original. Affected by this, the oil temperature difference between the two ends of the oil baffle is as high as 2.42℃. Secondly, this paper conducted a short-circuit temperature rise test on the test prototype, and selected the different axial heights (18%, 72%, 96%, etc.) of the windings on both sides of the transformer as the characteristic sample points for embedding temperature measurement fiber for real-time temperature monitoring. After comparison, it is found that the simulation calculation is basically consistent with experiment data, and the relative error of the two is less than 3.3%.