High-altitude regions significantly affect the discharge characteristics of long air gaps, presenting challenges for the design of external insulation in power transmission systems. This study investigates the optical development and current characteristics of an 8 m rod-to-rod air gap at an altitude of 3400 m, employing high-speed imaging with a frame rate exceeding 100,000 fps and synchronized current measurement techniques. The experiment captures the detailed progression of leader inception, propagation, and breakdown under high altitude conditions. In addition, the 50% breakdown voltage is obtained, revealing the influence of altitude on discharge behavior. Results indicate that high altitude leads to significant alterations in the discharge process, with variations in both optical and current characteristics compared to sea-level conditions. These findings provide essential data for understanding the underlying discharge mechanisms and validating simulation models, contributing to the development of more accurate predictive models for high-altitude discharge behavior. The results offer valuable insights that can guide the optimization of external insulation design in power transmission systems operating in elevated regions, ensuring more reliable performance in challenging environmental conditions.