For revealing the microphysical mechanism of the formation and development of the cloud-ground lightning channel, the radial structure and optical radiation characteristics of the cloud-ground lightning channel were studied. We carried out field observation experiments in the Qinghai Plateau region using a slit-less high-speed spectrograph. The clearly visible channel core was recorded in a cloud-ground lightning, and a weak luminescent region was found between the outer edge of the channel core and the external luminescent channel. Based on the spectral observation results, the optical radiation characteristics of the first return stroke and the third subsequent return stroke were compared and analyzed. The corona sheath model of the lightning channel was verified experimentally, the location of the connection point was determined. The estimated striking distance of the two return strokes is 57 m and 53 m respectively, and the strongest point of the return discharge is confirmed at the connection point. It can be inferred that in the initial stage of the return stroke, the cloud ground lightning return stroke channel consists of the channel core , the weak luminescence region and the outer corona sheath from the inside to the outside, that is, the charge distribution along the radial direction of the lightning channel is uneven. The light radiation characteristics of lightning channel are closely related to the intensity and duration of discharge.