In order to achieve accurate temperature monitoring and status detection of the transformer cooling radiator, the safe and efficient operation of the oil-immersed transformer cooling system is further realized. In this paper, the radiator of an oil-immersed transformer is the research object. Based on heat transfer, fluid mechanics, and energy conservation, the three-dimensional numerical model of the oil-natural air-natural (ONAN) and oil-natural air-forced (ONAF) cooling modes are established respectively. The CFD simulation verifies the validity of the model and analyses the heat dissipation of the radiator, and the oil exponent of the thermal parameters of the transformer is revised. In addition, the characteristics of the temperature rise of the radiator before and after the failure of the transformer cooling fan are also studied in this paper. In the process of the temperature rise of the radiator, the local area of the excessively high oil temperature is located, which provides a reference for the maintenance and efficiency improvement of the transformer cooling system.