With growing environmental concerns, natural ester transformers are widely used due to their excellent properties and good biodegradability. However, their thermal characteristics under extreme ambient temperatures remain unclear, limiting their widespread adoption. Herein, a Numerical simulation model is used to simulate and evaluate the temperature distribution, hot-spot temperature and thermal stability of natural ester transformers. A mineral oil transformer is used for comparison. The results indicate that the hot-spot temperature of the natural ester transformer is always higher than that of the mineral oil transformer under the same heat dissipation structure. With the increase of ambient temperature, the variation trends of their hot-spot temperatures initially decrease and then increase, and the minimum hot-spot temperatures appear around the -10°C ambient temperature. The hot-spot temperature increases when ambient temperature continues to go up. However, the hot-spot temperature of the natural ester transformer remains within the limits specified by the IEEE standard under the extremely high ambient temperature (52.2°C). In contrast, the mineral oil transformer exhibits localized overheating in the windings, in which the overheated zone accounts for approximately 50% of the total winding. Actually, when the ambient temperature exceeds 45°C, the hot-spot temperature of the mineral oil transformer reaches its limit, while that of the natural ester transformer remains within its safe limit. The results demonstrate the superiority of natural ester transformers when used at high ambient temperatures, while also providing a reference for the design and application of natural ester transformers under extreme ambient temperatures.