Oil immersed transformer is an important equipment of power system, whose fault is often caused by the aging of insulation. In actual operation, accurate calculation of the temperature field of transformer winding hot spot temperature in particular, has a very important significance for stabilizing the operation of power grid and extending life of the transformer. In order to accurately calculate the temperature field of oil immersed transformers, a hybrid method which is based on comparison of respective advantages of the finite element method and finite volume method is proposed； it takes a transformer of 110KV / 31.5MVA as example, using mixed method of finite element method and finite volume method, the three-dimensional temperature field of transformer is calculated. By comparing the figures of top oil temperature of transformer monitored by the operating transformer and the figures calculated by mixed method, the margin error of the hot spot temperature is only 1.6 ℃, proves that this hybrid method can accurately calculate the internal temperature field of oil immersed transformer.