Abstract: In the central and western regions of China with an altitude of over 2000m, the shielding sphere fittings with a curvature radius of 0.25~1m are widely used in the valve hall of UHVDC converter stations. The sphere-plane gap formed with the ground and wall directly affects the insulation safety and economic cost of the whole transmission system. Therefore, it is of great theoretical significance and engineering value to study the discharge conversion characteristics of large size sphere-plane gap at a high altitude. In this paper, the positive switching impulse voltage discharge tests of sphere-plane gaps with curvature radius of 0.30, 0.65 and 0.80m are carried out under the gap distance of 3m at the Qinghai UHV test base at an altitude of 2200m. The photoelectric physical parameters such as instantaneous optical power, spatial electric field strength and high potential current at each discharge stage of the three types of electrode structures are compared and analyzed. The physical characteristic values of the initial injected charge, spatial distribution of electric field intensity, initial streamer length and stem temperature are calculated. The effect of gap structure on discharge conversion characteristics of large size electrode is studied. The analysis results show that when the gap distance is the same, the larger the electrode curvature radius is, the longer the initial streamer length is, the more space charge is generated, the faster the temperature rise rate of the stem is, and the streamer gradually becomes the dominant process in the discharge process. The research results of this paper lay a foundation for selecting suitable shielding structure for valve halls of converter stations in high altitude areas, effectively enhancing the impact insulation level of clearance operation and reducing the size of equipment.