A measure system for checking high reflectivity of the coating by cavity ring-down spectroscopy (CRDS) at IR wavelength has been set up. The measure precision could approximate to be 10-4 with a pair of high reflectivity concave mirror put aside to make a straight cavity. The ringdown curves have been gained in different cavity lengths,for change of cavity mode would cause different modulation depth curves. Then the reflectivity of the cavity mirror is calculated and the average square error is 2.25×10-4. Then the influence of the reflective measure precision is studied with adjusting the relative position and adding the diaphragm in optic length which could change cavity mode. The average square error is 2.55×10-5 in the adjusting range of 20 mm between two lens. The use of the diaphragm in optic length could improve the match of the cavity modes which would reduce the accord error of the ringdown time.