According to the time-resolved spectra of three lightning return stroke processes, the temperature of the arc core channel and the peripheral luminous channel is calculated with different methods, and the evolution of channel temperature with time in the process of current decay is studied. The results show that the temperature of the core current channel is about 4000~5000 K higher than that of the peripheral luminous channel. The decrease of the channel temperature with time is very slow compared with that of the discharge current after the peak value occurs. Up to about 400 μs after the peak current occurs, the channel temperature still stays at about 20000 K, and the heat effect resulting from high temperature for such a long time is the main source of most lightning disasters.