This study analyzes the response changes of the Medium Resolution Spectral Imager II (MERSI-II), which is an optical instrument on the Fengyun-3D satellite, during different periods since liftoff. The study was based on the Dunhuang radiometric calibration site. The solar reflective channels, assessed using the reflectance-based method, showed a degradation of MERSI-II. The degradation rate of change was found to be higher in the beginning of the mission and to be decreasing with time. The latest calibration results show that the decrease in instrument response between 440 and 900 nm in the near-infrared band was relatively small compared to the previous official calibration coefficient, with an average attenuation rate of ∼5.3  %  . It was observed that the MERSI-II shortwave channel had a large attenuation of 21%, especially in the 412 nm band. The calibration method and parameter processing of the reflectance-based method are introduced in detail. The accuracy of the calibration process was tested using Moderate Resolution Imaging Spectroradiometer, an internationally recognized high-precision satellite instrument, which determined the calibration error to be <4  %  . The experimental results contribute to the improvement in the accuracy of satellite observation and the design of subsequent equipment.