Experiments are carried out in a 5 m rod-plate gap under positive lightning impulses, and the discharge voltage, discharge current and video image are recorded synchronously. According to characteristics of the discharge current at different stages, three main discharge phases are identified, which are the first streamer discharge, the second one and the leader discharge. The influences of impulse amplitude and electrode shape on the characteristics of each discharge stage are studied. The results show that the space charge generated by the first streamer discharge is positively correlated with the voltage amplitude and the radius of curvature of the electrode, and its suppression makes the average value of the initial delay of the second streamer is approximately constant. The average velocity of the second streamer is 700~1 060 cm/μs, which increases with the voltage amplitude, and decreases with the radius of curvature. The average speed of the leader is 29~111 cm/μs, which increases linearly with the voltage amplitude, and decreases with the curvature radius of the electrode.