Combustion driven chemical laser system has a number of key species in ground states such as DF molecules which can characterize the performance of the combustor of chemical lasers. Additives such as SF6, NH3 and H2O may be added into chemical lasers as collisional particles in order to control the relaxation processes of vibrational-rotational excited states of hydrogen fluoride molecules. There are also some key intermediate species such as NF(a) which could interact with vibrational-rotataional excited states of hydrogen fluoride molecules via near-resonant energy transfer processes. However, the absorption coefficients of these key species are generally very small. In order to obtain the absorption spectrum of the ground state of these key species, an off-axis cavity enhanced absorption spectroscopy apparatus was established. The apparatus consisted of the light source, the optical resonator and the photoelectric receiver, wherein the resonator was built within a vacuum chamber. In order to verify the performance of the device, the absorption spectra of trace amounts of ammonia and water vapor were measured. The results showed that the noise equivalent absorption coefficient of the device reached 1.6×10-8 cm-1. The experiment results show that the cavity enhanced absorption spectrometer can be used to obtain the number densities of those key species in HF chemical lasers.