Abstract: According to the analysis the changes of the cavity temperature and particle population ratio in a continuous wave DF laser, it was pointed out that the quantum number of maximum small signal gain and shortwave direction quantum number of zero small signal gain will move to high rotational states during with cavity temperature and particle population ratio increasing, and they must be corresponding to a certain range of cavity temperature and particle population ratio. The integer quantum numbers of zero small signal gain are difficultly obtained under measured spectrum. The conditions were given that small signal gain of shortwave direction cut-off quantum was greater than zero and small signal gain of reduce one shortwave direction cut-off quantum was less than zero. Combining with the same cavity temperature range of three spectral bands, calculation based on spectral data of DF laser that the cavity temperature range was 305-368 K, the particle population ratio of 1P band, 2P band and 3P band were 1.77-2.09, 1.65-2.05, 1.19-1.72, respectively.