Chemical oxygen iodine Laser (COIL) power is restricted by the reflectivity of COIL mirrors who are important parts of COIL, so the reflectivity of COIL mirrors must be tested precisely. The precision of measurement system is high for the high reflectivity of COIL mirrors based on cavity ring-down spectroscopy, the reflectivity for an 45° optical mirror of COIL mirrors is 99.979%. Uncertainty of measurement system for the high reflectivity of COIL mirrors are based on cavity ring-down length, cavity mirrors ring-down time and tested mirrors ring-down time. By studying the vertex of curve ring-down and tests repetition, it is found that vertex of curve and tests repetition are sensitive. When ring-down curve is managed, the vertex of curve is properly choosed. Uncertainty analysis of measurement system for the high reflectivity of COIL mirrors has been done, expanded uncertainty is 0.0152%.