The upward transmission process of the lightning return stroke current is a key carrier for characterizing the channel radial expansion during the transition stage 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 of two lightning return strokes at the Canton Tower during the upward current propagation, and for the first time analyzed the optical radiation of the channel’s radial expansion during the upward propagation of the return stroke current, as well as the distribution characteristics of the temperature and electron density along the channel in 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 dominated by a positive streamer accompanied by intense neutral radiation at a speed of approximately 10⁵ m/s. This expansion neutralizes the negative charges deposited in the leader corona sheath, leading to 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 an intense energy exchange between the channel core and the surrounding corona sheath. The temperature of the channel core decreases with increasing channel height, and the rate of decrease gradually slows down. For the surrounding corona sheath, the variation of its temperature with channel height differs at different 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, the corresponding electron density of the channel core also increases with the channel height; when the electron density of the corona sheath decreases with the channel height, the corresponding electron density of the channel core remains basically stable along the channel height.