In order to analyze the air gap flashover characteristics of switching impulse voltage of fittings in Xinsong converter station at the altitude of 2 400 m, according to the fittings structure size of valve hall in Chuxiong converter station, this paper selects Φ750 mm/Φ880 mm cap, Φ1 200×300 mm/Φ1 600×300 mm/Φ1 900×300 mm grading ring and Φ1 100 mm/ Φ1 600 mm/ Φ2 000 mm equalizing ball, all in typical size, and carries out an experiment in Kunming UHV base at the altitude of 2 100 m. On this basis, this paper checks the air clearance of transmission side valve hall in Xinsong station and presents the discharge voltage curves for different altitudes after altitude correction. The study of air gap discharge characterstics and altitude correction provides a reference for practical engineering.