Cavity Enhanced Absorption Spectroscopy (CEAS) technology is an important part of high sensitive spectroscopy, which has the advanced characteristics of relatively simple apparatus, high sensitivity, and strong environmental adaptability. Using a 2.0 μm tunable diode laser as the light source, combined with CEAS, a device for measuring the reflectance of the lens was built, and the absorption spectrum of CO₂ gas at 5 001.49 cm^-1 was used as the research target. The path length absorption cell and the known concentration of absorption gas have been calibrated for the reflectivity of the same pair of high reflectivity lenses. The first calibration method uses a multi-pass cell with a known path length as a reference cell. In this calibration, the absorptions from the resonant cavity and the multi-pass reference cell are measured simultaneously and compared to deduce the effective absorption path of the cavity enhancement system. Then the mirror reflectivity is obtained from the relationship between the mirror reflectivity and the effective absorption path. In the second calibration method, the integrated absorbance of a CO₂ transition in a mixture with known concentration are measured for calibration. The mirror reflectivity is calculated by the relation of the integrated absorbance, the gas molecule number density and the line intensity of the selected CO₂ transition. Finally, the two methods are compared for the calibration results of the lens reflectivity. The results show that the ratio of the integrated absorption area of the signal measured by the integrating cavity to the reference cell in the first method is 10.5, and the effective absorption path of the integrating cavity and the reflectivity of the lens are 302.65 m and 99.85%, respectively. The concentration of CO₂ gas in the atmosphere is 0.037 3%, which is consistent with the actual atmospheric CO₂ content. The advantage of this method is that it is not affected by sample concentration, but because of the introduction of a new reference cell, the pressure and temperature of the gas in the two cells need to be kept the same, so this method is suitable for an open cavity structure. In the second method, the absorption spectrum of CO₂ is measured at 5 001.49 cm^-1, the molecular number density N of CO₂ gas is 9.099 101 5 molecules·cm^-3, and the line intensity of the line in the Hitran database is 3.902×10^-22 cm·molecule^-1, the reflectance of the lens is calculated to be 99.84%. The advantage of the above method is that the structure is simple, but the molecular number density of the gas needs to be known, so the error of concentration and pressure will affect the calibration of the reflectivity of the cavity mirror. These two methods can accurately calibrate the reflectivity of the cavity mirror, and the corresponding suitable method can be selected as a reference in actual application.