Ultra-low loss mirror (super-mirror) is an important optical element for high-quality optical cavity and optical sensitive measurements in many fields, and the measurement of mirror losses on 10-6 level is an important issue. We have effectively mode-matched the incident beam to the TEM00 mode of cavity by scanning cavity length and sweeping laser frequency near the TEM00 mode of cavity. An optical switch triggered by a hysteresis comparator was employed to cut off the incident laser beam and the cavity ring-down (CRD) signal was obtained. We have measured a batch of super-mirror with losses on 10-6 level. The precision of this system is better than 10-6, and the highest reflectivity is (99.99914±0.00004)% . The results indicate that the losses of super-mirrors are diverse even if the super-mirrors are from the same batch and stored in the same environment.