For a diffuser which is used as the light source carrier for in-orbit calibration of an optical remote sensor, to use a ratioing radiometer to monitor the variation of its reflection performance in orbit is an effective means. In order to meet the inversion requirement of quantitative products, the absolute radiation in-orbit calibration system for a new generation of spaceborne High Resolution Imaging Spectrometer (HRIS) should be designed to use the sun as its light source. The long-term tracking measurement results of a foreign in-orbit calibrator show that the reflectance of the diffuser is changing all the time. Moreover, the shorter the wavelength is, the greater the change is. However, the remote sensing of ocean color elements just concentrates on the wavelength region close to UV. To solve this problem, a spectrum ratioing radiometer is designed. Its radiation detection performance is analyzed theoretically and measured experimentally. The results show that its linear correlation coefficient is better than 0.9991 in the whole dynamic measurement range and its spectrum signal-to-noise ratio under the in-orbit irradiation input condition is better than 100. It can meet the calibration precision requirement of the reflection performance of the spaceborne diffuser for a spaceborne HRIS.