On the basis of thermoelectricity analogy, with the features of the structure and heat transfer of oil - immersed transformer, this paper established the complete thermal model and the differential equation for predicting the lop oil and hot - spot temperature. According to the temperature rise of 250 MVA oil - immersed transformer calculated by MATLAB, the calculated value of oil - immersed transformer load guideline was compared to verify the validity and reliability of the proposed thermal model.