The cavity ring-down method is currently the only method for measuring ultra-high reflectivity (reflectivity is greater than 99.9%) of optical elements. An optimal extraction method for the relationship between laser signal intensity and time in cavity ring-down method was introduced, and a test system for ultra-high reflection ratio of optical elements based on cavity ring-down method was designed. The data of cavity ring-down curve were divided into five sections by piecewise exponential fitting, the R² (R-square) and root-mean-square error (RMSE) values corresponding to the index fitting results of each section were compared and analyzed, and the ring-down time of each index fitting was calculated. The experimental results show that the results obtained from 40%~60% of the cavity ring-down curve data are closest to the real value, and the reflectivity of the corresponding cavity mirror is 99.988 977%. Finally, by comparing with the reflectance of the cavity mirror, it is shown that this data fitting method can effectively measure the reflectivity of the cavity mirror and reduce the error caused by the experimental data.