Abstract: The highly reflective (HR) mirrors with high-performance are widely employed in mid-infrared (mid-IR) laser systems. The manufacturing of mid-IR HR mirrors with high reflectivity requires techniques to precisely measure their high reflectivity. In this paper, a continuous-wave cavity ring-down (CRD) experimental apparatus in the 2.7−3.0 μm spectral range is established based on a quantum-cascade laser for high reflectivity measurement. By precisely optimizing the laser wavelength within the reflection band of the mid-IR HR mirrors, analyzing the influence of water vapor absorption on the ring-down time and reflectivity measurements, and comparing the reflectivity results measured under ambient air in clean-room laboratory and under nitrogen purging, the accurate measurement of high reflectivity is achieved at the 2.7−3.0 μm spectral band with an absolute reflectivity measurement accuracy of below 2×10⁻⁵ for about 99.95% reflectivity. The experimental results demonstrate that by setting the laser wavelength precisely to 2.9 μm and employing equal lengths of initial and test ring-down cavities (RDC) to avoid the influence of water vapor’s absorption lines, the reflectivity measurement for the 2.7−3.0 μm spectral band can be performed under normal clean-room laboratory air, without the need of nitrogen purging.