Temperature is one of the crucial parameters reflecting the energy and current transfer characteristics in the lightning discharge channel. According to the spectra of eight lightning return strokes recorded simultaneously by two high-speed slitless spectrographs with different time resolutions, the spectral-structure and temperature evolution of the return stroke channels over time was quantitatively analyzed. Different from the previous report, one of the spectrographs have recorded the ionic lines in the spectra within approximately 200 microseconds during the return stroke. The ionic line intensity decayed rapidly with time as the current declined, while the atomic line intensity decreased more slowly. The spectral-structure evolution characteristics indicate that the ionic lines in the spectra existed throughout the discharge current process (including the continuing current (CC) stage). Additionally, it further suggests that the ionic line intensities are associated with the discharge currents and that their radiation mechanism is closely related to the collision excitation under the action of strong currents. The temperature calculated by the ionic lines can reflect the thermodynamic properties of the current-carrying channel. The temperature calculated using atomic lines is significantly lower than that calculated by the ionic lines in the same spectrum. The radiation mechanism of the atomic lines differs from that of the ionic lines. Compared with the decay of the ionic-line intensity, the decline of the channel temperature calculated by ionic lines is slower. The temperature calculated by atomic lines shown the similar feature, and it is even basically unchanged in the CC stage. This property reflects the persistent heating effect of the current.