Abstract: In order to precisely calibrate the absolute spectral responsivity of detector from 115 nm~400 nm based on cryogenic radiometer, an optical chopper used in vacuum was developed. The optical chopper consisted of blade, rotation axis, servo motor, U-shaped photoelectric switch, cooling module, bracket and controlling circuit could be used to modulate the weak vacuum ultraviolet-ultraviolet (VUV-UV) radiation signal to the alternating radiation signal which the frequency was known in vacuum cryogenic environment, and measured by lock-in amplifier. The experimental results show that the stability of the optical chopper is ±0.05 Hz at the frequency of 80 Hz, which meets the requirements of absolute spectral responsivity calibration of detector at 115 nm~400 nm in the vacuum environment of 10⁻⁴ Pa.