Air-cooling technology can effectively reduce the operating temperature of transformers, but the cooling effect of transformers in extreme temperature environments is not significant. The combination of nano-modified transformer oil can effectively solve the above shortcomings. Based on the finite element method, this paper established a numerical model of the combination of air-cooled and nano-modified mineral oil to enhance the heat dissipation of transformer in extreme environment, and studied the heat transfer characteristics of conventional transformer oil and nano-modified transformer oil in extreme temperature environment (313.15 K). The results show that the influence of ambient temperature on the average temperature of the winding is limited when the heat value of the winding is unchanged. The influence of different types of transformer oil on the average temperature of air-cooled radiator is not obvious. With the increase of the heat transfer coefficient of the air-cooled radiator, the hot spot temperature of the nano-modified transformer oil can be reduced by 8.0 K at most compared with that of the conventional transformer oil. The results of this study can provide valuable guidance for the design of transformer cooling with air-cooled nanometer modified transformer oil in extreme temperature environments.