Chemiluminescence is an important method of measuring parameter in optical cavity flow field for HF/DF chemical laser. In the process of increasing H₂ flow rate, the changing of the chemiluminescence in visible range in the optical cavity flow field of discharge driven HF laser was observed and recorded, and the spectrum of the chemiluminescence on the optic axis was measured. The component particle of the gas in the optical cavity was analyzed theoretically, and the high order overtone spectral line of HF molecule was computed. It was found that the spectral line of the transition from 3 order vibration excited state to ground state was in infrared region. The measured spectrum of the chemiluminescence and the spectral line of the particle was compared, discovering that the yellow-green chemiluminescence was formed by the radiation of the excited F, He, H, N, HF and other particle, which corrected the explanation in some classic literature that the yellow-green chemiluminescence was formed by the 3,4,5 order overtone radiation of HF molecule. This further explains the color forming of the chemiluminescence on the optic axis.