Medium Resolution Spectral Imager (MERSI) is the key imaging sensor on board Fengyun-3 (FY-3) ,the second generation polar-orbiting meteorological satellites of China, currently operating on both FY-3A and FY-3B satellites. The MERSI is a major advance over the previous generation of sensors in terms of its spectral, spatial, radiometric and temporal resolutions. It has 20 spectral bands: 19 reflective solar bands (RSB) with center wavelengths from 412 to 2100 nm and 1 thermal emissive bands (TEB) with center wavelength 12000nm, making observations at two spatial resolutions: 250 m (bands 1–5) and 1km (bands 6-20). MERSI is a cross-track scanning spectro-radiometer with a wide field-of-view, providing a complete global coverage of the Earth within one day and can help to advance our scientific understanding of clouds, radiation, atmosphere, land and ocean. Both the MERSIs went through extensive pre-launch calibration and characterization at various levels. Its in-orbit calibration and characterization tasks are performed using its on-board calibrators (OBCs) that include a Visible Onboard Calibrator (VOC) and a v-grooved flat panel blackbody (BB). In this paper, we present an overview of MERSI calibration and characterization activities, methodologies, and performance monitoring. Key issues discussed in this paper include in-orbit efforts of monitoring trend of the detectors' response, tracking the sensor's degradations, and assessing calibration consistency between the VOC calibration monitoring, the China Radiometric Calibration Sites(CRCS)calibration and cross-calibration with the Terra/MODIS. The results from these three calibrations are consistent that there is significant degradation of shorter wavelength bands (<500nm) of MERSI with a degradation of more than 15% and the longer wavelength bands are relatively stable with the degradation rate less than 5% during the three-year. The instrument performance monitoring also shows that the calibration slope of FY-3A/MERSI two SWIR bands has frequent fluctuation because of random jumping of MERSI electronic gain on orbit. The overall uncertainty in the MERSI top-of-the- atmosphere (TOA) apparent radiances or reflectance is less than 5%, which warrants successful retrievals of a lots of Level 2 products of atmosphere, land and ocean and enhance the application of the MERSI data. The on-orbit long-term performance of the FY-3A MERSI should offer a good insight of the new instruments onboard the successive FY-3 satellites. These experiences and lessons learned from MERSI monitoring can play major roles in the design and improvement of future sensors and also provide important reference for retrospective recalibration of MERSI's historical data.