Continuing current is an important sub-physical process of lightning discharge. It refers to the process in which the local charge center in the thunder cloud discharges to the ground through the original channel after the return stroke. It is also usually overlapped by the M-component, which is the phenomenon that the brightness of the glowing channel increases suddenly. Since the continuing current was discovered in the 20th century. Many kinds of research were made by domestic and foreign researchers. The present studies mainly reveal the macroscopic characteristics of the discharge and luminescence using electromagnetic and optical observations. There is a lack of studies on the microcosmic luminescence information and the physical characteristics used by spectral observation. There are few studies about the temperature in the discharge channel of the continuing current. However, the temperature is not only a basic parameter to analyze the physical properties of the continuing current discharge channel but also a concerned parameter to prevent lightning disasters caused by the continuing current. Based on the spectra of a first return stroke and the following continuing current process overlapped with three M-components for cloud-to-ground lightning recorded by a slit-less high-speed spectroscope, the spectral evolution properties during the entire discharge process have been analyzed. The temperatures in the channel core and the corona sheath have been calculated, and the variations of both along the channel height have been studied. The results show that in the stage of the return stroke, the channel optical radiations are mainly the NⅡ lines with higher excitation energy. In the continuing current process, the channel optical radiations are mainly the NⅠ and OⅠ lines with lower excitation energy. The intensity of the ionic lines is strongest at the initial stage of the return stroke, while the intensities of the Hα and the neutral atomic lines are strongest at M1, and the continuum spectrum is strongest at M2. Four lines of OⅠ 777.4, NⅠ 746.8, 821.6 and 868.3 nm in the near-infrared band were observed throughout the discharge process. During the continuing current, the temperatures in the channel core are 42 060~43 940 K, which are 6 020~7 900 K higher than the temperature in the channel core of the corresponding return stroke. The temperatures in the outside corona sheath are 16 170~20 500 K. The temperatures of the channel core, and the corona sheath both remain unchanged with time. The temperature of the channel core decreases with the increase of the channel height, while the temperature of the peripheral corona sheath increases with the increase of the channel height.