The local temperature of Castresin drytype power transformer overheated can largely reduce the service life of drytype power transformer. In this thesis, a SC9 castresin drytype power transformer whose foiltype LV winding has one air duct and layertype HV winding has no duct is investigated. A simplified 2D axisymmetric physical model of drytype power transformer is established. A finite element analyzing software ANSYS is used to simulate the steadystate temperature field of the castresin drytype power transformer. This thesis also establishes and analyzes the drytype transformers’ equivalent thermal circuit model by using the analytic method of thermal circuit. The transient temperature field and the steadystate temperature field of the drytype power transformer are respectively solved by using the method of thermal circuit based on measured surrounding air temperature, furthermore, a computer program based on Visual Basic is developed.