Abstract: A three-dimensional fluid-temperature field coupling analysis method of transformer based on finite element method is introduced in this paper. Firstly, based on the actual size of the transformer, by simplifying the three-dimensional modeling of the various parts of the transformer in Solidworks, and assembling according to the actual matching relationship, the overall three-dimensional model of the transformer is obtained. Based on the use of fluid continuity equations and Reynolds equations, the boundary conditions of the finite element calculation of the transformer cooling system are determined. Then based on the fluid network theory, the Fluent software is used to perform finite element analysis on the overall oil flow of the transformer cooling system, Based on this, the winding temperature field and fluid field are calculated. At the end of the article, a SFJZ9-150 000 kVA/220 oil-immersed air-cooled (ONAF) transformer is used as an example to calculate the transformer winding temperature distribution and the overall oil flow distribution of the transformer, and compare the winding temperature calculation results with the actual test results of the manufacturer. the correctness of the method is verified.