Remote sensing instrument must be calibrated the optics reflectance of the instrument in vacuum conditions before lauched. To obtain the spectral reflectivity of less than 200 millimeter diameter optical element in vacuum, the reflectance of test system is constructed. The reflectivity of the test system is consisted of a light source, the Seya-Namioka vacuum visible monochromator, the sample room as the main structural and electronic system components. It describes how the optical systems and electronic are designed. The monochromator worked band is from 400nm to 780nm, spectral resolution is 0.5nm. Dual optical compensation method is used to eliminate the source of time drift, improve the measurement accuracy with phase-locked weak signal amplification method. The system used a lock-in amplifier according to the technique using the principle of coherent detection of the modulated optical signal which is multiplied with the reference signal processed, by using the optical integrator the signal will be smoothed sended to A / D converter. To ensure the precision measurement deteced, the phase-sensitive detector function can be adjustable. The output value is not more than 10mV before each measurement, so it can be ensured that the stability of the measured radiation spectrum is less than 1 percent. The reflectivity of the test system results is shown that the wavelength accuracy is 0.1nm, and the wavelength repeatability is 0.05nm, it can achievehigh-precision measurement of optical components under vacuum body.