To calibrated the reflectivity of the mirrors in optical cavity, two methods of the calibration of mirror reflectivity, which are through a known concentration of an absorber and the difference of Rayleigh scattering, respectively, were introduced and compared. The reflectivity of the same pair of the highly reflecting mirrors were calibrated by these two methods. The reflectivity of the highly refecting mirrors are both 0.99962 to 0.99990 between the spectrum range of 359~380 nm, the linear correlation is 0.998, the results showed the good correlation. The atmospheric NO2 concentrations were simultaneously measured by the cavity enhanced absorption spectrocopy instrument and a commercial nitrogen-oxide analyzer to verify the accurate of calibration of the mirror reflectivity.