Abstract: Streamer-to-leader transition is the most important point of energy transformation during the discharge in a long air gap. Aiming at the large number of sphere-plane long air gaps that are composed of a spherical shield and surrounding earthed plane in the valve hall of converter stations, it is of great academic significance and engineering value to study the transformation process and mechanism. In this work, a sphere-plane long air gap discharge test under positive switching impulse voltage was carried out. A combined photoelectric detecting system was used to collect the photoelectric information such as instantaneous optical power and spatial electric field strength and measure duration t_a of streamer-to-leader transition during discharge. Compared the measured duration t_a with the calculated duration t_s obtained by the classical Gallimberti streamer-to-leader transition model and found that the classical model was suitable for the discharge analysis of small-size electrodes with only one stem generated and less charge injected. A streamer-to-leader transition correction model was established considering the air gap discharge convective diffusion and stem geometric characteristics. The stem number n and stem radius r were determined according to initial streamer injected charge Q_z and discharge current I_z calculated by the electric field jump. The correction duration t_sz of the streamer-to-leader transition was simulated and was compared with measured duration t_a and calculated duration t_s. The analysis results show that streamer-to-leader transition correction model considering the effect of stem number and initial injection electric charges increasing of large diameter electrode can significantly improve calculation accuracy. The research results in this paper lay a foundation for the establishment of a more comprehensive sphere-plane long air gap discharge leader initiation model in long air gap with a large diameter electrode in the future.