Abstract: Approximate condition of reflectivity measurement with cavity ring-down method is analyzed,misadjustment characteristic of confocal cavity is discussed and influence of cavity mirror's reflectivity on ring-down signal is calculated.In order to lessen metrical precision from diffraction losses of OPO laser,based on the above analyses,a reflectivity measuring system for single wavelength is set up with cavity ring-down method,whose ring-down cavity is confocal cavity.Experimental measurement of reflectivity of cavity mirror and inserted mirror is carried out.The measurement RMS in linear cavity is less than 6×10-6.Results show that cavity ring-down method is only for higher reflectivity,and metrical precision can be improved by using logarithm transform of oscillograph in adjustment of the optical cavity.