In order to directly measure the vacuum ultraviolet spectrum reflectance of spherical mirror a measurement system was established. The system consists of monochromatic light source with deuterium lamp of magnesium fluoride window and Seya-Namioka concave grating monochromator, optical compensator with reflection modulator and reference detector and the receiving system with 80 mm diameter fluorescence integrating sphere and precision turntable. The optical compensation eliminates the instability in monochromatic light source, and the fluorescence integrating sphere eliminates the impact of light spot sizes in the two measurements, and reduce the system energy loss. The system measures the spherical mirror reflectivity in 115~180 nm, and the measurement result show that the repeatability is less than ±0.3%, and the relative uncertainty is less than 1.3%. The spherical mirror reflectivity achieves high-precision measurement.