The loss measurement error caused by difference between the test cavity length and initial cavity length and by detection coupling scheme are analyzed. Measurement results show that even in class 1 000 clean-room laboratory, at 635 nm wavelength, the air loss coefficient is approximately 10 ppm/m. When the length of the test cavity is approximately 20 cm longer than that of the initial cavity, the air loss caused approximately 2 ppm error to the measurement of the optical loss of the high-reflectivity optics. For CRD signal detection, the use of Alternating Current (AC) coupling scheme distorts the measured ring-down signal waveform, results in loss measurement error of approximately 2 ppm as compared to the Direct Current (DC) coupling scheme. The analysis and measurement results show that for the measurement of the optical loss of ultra-high reflectivity optics with CRD technique, the same lengths for the initial cavity and test cavity should be used and the DC coupling detection scheme with sufficient bandwidth should be employed.