Abstract: In order to study the technique for spectrum reflectivity measurement of calibrated optic system in vacuum and low-temperature environment, a small test system which was applied to measure the spectrum reflectivity of the off-axis parabolic mirror in situ was built. The system consists of solar simulator, collimating lens, folding mirror, off-axis parabolic mirror, spectrometer and control system. The measurement was carried out in atmospheric environment and simulated space environment,respectively. The measured spectrum was from 350nm to 950nm. The average spectrum reflectivity of the mirror was 0.882 in atmospheric environment, it was 0.863 in simulated space environment. When the mirror went back in the atmospheric environment, the average spectrum reflectivity of the mirror was 0.883. In the test, the measurement repeatability was 1.1％, the uncertainty was less than 2.4％. The test results show that the spectrum reflectivity of the off-axis parabolic mirror decreases in simulated space environment, and increases when the environment is back to the atmospheric environment.