Abstract: Currently, the highest altitude for which ultra-high voltage (UHV) AC transmission lines have been designed in China is 4500 m. However, there is still a lack of experience in the selection of conductors for UHV AC lines in regions above 4000 m. To address the challenges of power transmission systems in extremely high altitude areas and ensure the reliable operation of UHV AC transmission lines, a study on the selection of conductors for 1000 kV UHV AC transmission lines at altitudes between 4000 m and 4500 m was conducted based on the electromagnetic environment limits. Using the calculation methods verified by engineering practice in the "Power Engineering Design Manual", the study evaluated different models and configurations of conductors for surface electric field strength, radio interference, and audible noise. An annual cost comparison of different conductors was also performed. The results indicate that for 1000 kV UHV AC transmission lines at altitudes between 4000 m and 4500 m, the recommended conductor is the 10×JL1/G1A-630/45 aluminum conductor steel reinforced.