Spectrum carries important information reflecting atomic and molecular processes inside a light source. Accurate spectral diagnosis is vital in revealing the microscopic physical mechanism of the lightning discharge process. Spectral correction was applied considering the influence of atmospheric attenuation, grating efficiency, and camera response on the observed spectrum, thereby solving the problems of atmospheric attenuation in long distance lightning spectrum observations. Based on the restored spectrum, the temperature of the lightning return stroke channel was calculated by the ionic and atomic lines respectively. The result showed that corrected temperature at the initial stage of the return stroke, calculated by the ionic line, could reach up to 40,000 K, which was about 10,000 K higher than the values directly obtained from the observed spectrum. Atmospheric attenuation of the atomic spectral line in the nearinfrared band is relatively weak; therefore, atmospheric attenuation was inferred to have a relatively less effect on the channel temperature that was calculated by the atomic spectral lines. This work provided the attenuation ratio of the characteristic lines in lightning spectra with distance, and can used for more precisely quantitative investigation on the physical characteristics of the lightning process. It also has application value for improving the spectral diagnostic techniques on celestial body and other natural luminous process.