In order to study spectral properties of high temperature gas around the surface of aircraft in hypersonic speed, we propose a practical calculation method of spectral coefficient for high temperature gas molecules. By calculating the energy level population of high temperature gas molecules in the studied area of the aircraft, the broadening effect of spectral line is studied and the spectral parameter required is obtained based on the HITRAN database. Finally, the spectral radiation coefficient of the gas molecules is calculated by line-by-line method. The method can calculate the spectral coefficient of high temperature gas at corresponding wavelength effectively, and thus providing the important calculating parameters to analyze the aerodynamic flow field of hypersonic aircraft and the spectral radiation properties of the target. Furthermore, it can provide supporting data for the study of aircraft thermal protection and optical detection.