In order to achieved the coaxial optical designed unit for UV spectral radiation detecting and visible observation telescope, UV / visible beam splitter and four pieces of the quartz retarder were used, and a lock-in amplifier was used in the electical system to advance the accuracy of measurement. To ensure the precision measurement deteced,the phase-sensitive detector function can be adjustable. So the maximum residual polarization of the incident light for wavelength from 200 nm to 380 nm can be down to 3%. The output value of instrument is not more than 10 mV before each measurement , so it can be ensured that the stability of the measured radiation spectrum is less than 1%.