Abstract: Taking HF lasing as an example, line selecting performance of a single tube discharge driven subsonic CW HF/DF chemical laser was tested with grating dispersive cavity under the 1st order oscillating 1st coupling mode. The difference between the results of setting cavity axis at 2.0 mm and 0.5 mm downstream of hydrogen injecting orifice is dramatic. The results of the spectrum identification indicate that even for the subsonic chemical laser device, which owns only millimeter range flow direction's active length, line selected performance is greatly influenced by the cavity axis' relative position to the H2 injecting orifice. With the increase of this range, the transitions between the higher vibration states tend to be weaker, while the transitions between lower vibration-states more stro