Abstract: The relational expressions between gain coefficient, intensity of spectral lines and the temperature, population distribution in optical cavity were established, and the emission spectrum of a combustion-driven DF chemical laser was measured, calculation based on spectral data showed that the temperature in optical cavity was 381.4 K, the ratio of population in ν=1 vibration level to ν=0 level, population in ν=2 level to ν=1 level, and population in ν=3 level to ν=2 level were 0.60～0.62, 0.676 4, 0.71～0.74, respectively.