The research on the discharge characteristics and mechanism of air gaps has guiding significance for the insulation optimization design of UHV transmission projects. Leader discharge is the most important process in long air gap discharges. As an important development parameter, the accurate observation of the leader velocity is of great significance for the prediction of the breakdown characteristics of long gaps. In this paper, the switching impulse discharge test of ±800 kV UHV DC real-size conductor-tower gap was carried out. Two CMOS high-speed cameras were used to capture the optical images of the dis-charge development process from different directions. Considering the shooting elevation angle, based on the projection imaging principle of the camera, a universal three-dimensional reconstruction method of the discharge channel was derived, the three-dimensional structure of the leader channel development was obtained, and the axial velocity and three-dimensional velocity of the leader were further analyzed. The results show that the average value of the continuous axial development velocity of a typical leader channel is 2.09 cm/μs, and the average value of the three-dimensional development velocity is 2.23 cm/μs. The three-dimensional velocity is usually no more than 1.3 times the axial velocity. The average value of the axial development velocity of a typical leader-streamer system is 3.04 cm/μs, which is greater than the axial development velocity of the leader, indicating that the size of the streamer region at the leader head gradually expands with the discharge development.