The upward transmission process of the lightning return stroke current is a key indicator for characterizing the radial expansion of the channel during the transition from the leader to the return stroke. To clearly reveal the radiation mechanism and the spatio-temporal evolution of the discharge plasma parameters during this stage, this study utilized the high-resolution spectral data from two lightning return strokes at the Canton Tower. For the first time, the optical radiation of the channel’s radial expansion during the upward propagation of the return stroke current was analyzed, as well as distribution characteristics of the temperature and electron density along the channel in both the channel core and corona sheath. The results show that when the negative lightning return stroke current reaches the top of the corresponding leader channel, the channel undergoes radial expansion at a speed of approximately 10⁵ m/s, dominated by a positive streamer, accompanied by intense neutral radiation. This expansion neutralizes the negative charges in the leader corona sheath, resulting in a rapid attenuation of the longitudinal current in the channel core during the initial stage of the return stroke. During the return stroke current propagation, there is intense energy exchange between the channel core and the surrounding corona sheath. The temperature of the channel core decreases with increasing channel height, with the rate of decrease gradually slowing. For the surrounding corona sheath, the variation of temperature with channel height differs at various times during the initial stage of the discharge. The electron density of the channel core is significantly correlated with that of the surrounding corona sheath. When the electron density of the corona sheath increases with the channel height, that of the channel core also increases; when it decreases, the channel core’s electron density remains nearly constant along the channel height.