The invention discloses a method for accurately calculating a temperature field in an oil-immersed transformer. According to the method, a 3D physical model which completely corresponds to a real transformer in size and structure is built, scientific mesh generation is carried out on the physical model according to different characteristics of calculation methods, a finite volume method and a finite element method are combined for calculation, in other words, the finite volume method is used for calculating metal heat conduction of a winding and iron core portion and heat transfer in which fluid participates, and the accurate 3D calculation result of the temperature field in the oil-immersed transformer can be obtained; not only the temperature field of a winding or an iron core but also the temperature field of oil flow in the transformer, the temperature field of cooling fins and the temperature field of a transformer box can be accurately calculated, and the method has important and comprehensive referential significance in both design of transformers and actual operation of the transformers.