The spectral data of a lightning discharge process are captured by a high-speed slit-less grating spectrograph. Based on the plasma spectrum theory, the lightning return stroke channel temperature is estimated according to the spectral information of the different bands. The results show that the average temperatures of the lightning return stroke channel estimated by the Boltzmann plot method are 43270, 17660 and 17730 K, respectively, when the spectral line group of the single ionized nitrogen atom (NII), neutral oxygen atom (OI) and neutral nitrogen atom (NI) are selected respectively. The average temperature of the lightning return stroke channel is 24770 K, which is estimated based on the Saha-Boltzmann plot method by using NII and NI spectral line groups. Based on the lightning channel corona sheath model and the spectral radiation theory, it can be inferred that the temperature obtained by using only NII spectral line group should be the temperature of the lightning return stroke channel core, and the temperature obtained by using only NI or OI spectral line group should be the temperature of the corona sheath around the lightning return stroke channel core. Using both NII and NI spectral line groups, the obtained temperature should be the average temperature of the entire channel section (including the channel core and corona sheath) in the exposure time.