The hottest spot temperature (HST) of windings of oil-immersed transformer is an important factor that affects load capacity and operation life of transformer, and is closely related to the transformer load, top oil and environment temperature. Higher hot spot temperature, when operating at high temperature and overload, may lead to transformer failure, which will affect the normal operation of the power system. In order to accurately calculate the hot spot temperature of transformer winding, we construct the dynamic thermal model of transformer based on the theory of heat transfer and thermal-electrical analogy, and then, built a set of differential equations. By taking the effect of heat transfer coefficient of heat sink is different from inside to outside into consideration, we get the effective cooling area of transformer, and fix the thermal circuit model. Taking a 180 MVA-220 kV transformer of Tianwei transformer factory in Hefei as an example, we establish the thermal circuit model to calculate the hot spot temperature. Meanwhile, we establish a three-dimensional model, and get its internal temperature distribution based on Fluent simulation software. Finally, we carry out a temperature rise test on the main transformer of this main transformer winding, and the hot spot temperature data of winding is obtained. Combined with online monitored values, we compare and analyze the accuracy of thermal model and the model calculation results recommended by IEEE guidelines. The results show that the thermal circuit model with higher accuracy relative to the IEEE guidelines model, which proves that the fixed model can accurately calculate the hot spot temperature of oil-immersed transformer.