In order to measure the reflectivity of large-aperture and high-reflectivity optical elements in high energy laser transmission system, a precision measurement system for two-dimensional scanning of large-aperture optical elements was designed. The structure and working principle of the system were introduced, the factors affecting the measurement accuracy of the system were analyzed, and the influence of systematic error of the scanning system on the measurement accuracy was theoretically analyzed. The results show that the measurement error is 10^?6 magnitude when the horizontal deviation is 0.29 mm perpendicular to the beam propagation direction. When the variation of cavity length is small, the rotation axis deviation can be compensated and the system can be fine adjusted by adjusting the ring-down cavity mirror. By fitting the data of light intensity and time, the corresponding first-order exponential function fitting curve was obtained, and the ring-down time as well as the reflectivity was obtained by calculation. Through contrast analysis, this kind of error analysis method can more effectively measure the reflectivity of cavity mirrors and can reduce the error brought by the experimental data itself.