An onboard calibrator which consists of integrating minisphere and surveillance detectors for solar reflective bands is carried on the Medium Resolution Spectral Imager (MERSI). Based on the onboard calibration data from November 13, 2010 to April 11, 2013 of satellite FY-3B, it is proved that the degradation of MERSI on FY-3B is wavelength dependent. The annual degradation rate of channels on visible onboard calibrator (VOC) is less than 5.0%/year expect for Band 1(470 nm) which has noticeably annual degradation rate over 7.0%/year. The short wavelength bands (470～565 nm) of MERSI have an annual degradation rate performance more than 10.0%/year while the longer wavelength bands with annual degradation rate less than 4.0%/year during the past two years.