Abstract: The Sichuan-Xizang(Tibet)Plateau is a rich resource mine in China. With the development of energy in southeastern Tibet, it is expected to construct multiple ultra-high voltage direct current projects, and ultra-high voltage converter transformers are crucial equipment in ultra-high voltage transmission projects, and heat dissipation is the key to ensuring their safe operation. When the existing standards were formulated, the engineering did not face ultra-high altitude projects. To avoid the occurrence of improper selection of thermal design margin for transformers that may affect the safe and stable operation of equipment, the applicability of the standard can be further elaborated. This article mainly focuses on the theoretical analysis and simulation of the cooling capacity of the forced oil-circulating air cooler and the temperature rise of the top oil of the converter transformer at different altitudes. At the same time, the cooling capacity test was completed at different altitudes in the Environmental Climate Simulation Laboratory. Through comparison and verification of the experimental results and theoretical calculations, recommendations were made for modifying the cooling capacity of air coolers at different altitudes, namely, from 2000 m to 5000 m, and the temperature rise limit was reduced by 0.75 K for every 250 meters of altitude increase based on the 2000 m altitude. This study provides a reference basis for the design and standard revision of the cooling system for converter transformers in high-altitude environments, and iy is of guiding significance for the thermal design and reliability evaluation of converter transformers used in high-altitude environments.