Microwave Radiation Imager (MWRI) onboard the FengYun (FY)-3A/B satellites observes the Earth atmosphere at 10.65, 18.7, 23.8, 36.5 and 89.0 GHz with each having dual polarization. Its calibration system is uniquely designed with a main reflector viewing both cold and hot calibration targets. Two quasi-optical reflectors are used to reflect the radiation from hot load and cold space to the main reflector. Soon after FY3b was successfully launched in November 2010, evaluation of on-orbit calibration accuracy of MWRI was carried out. In this paper, CRTM was used to simulate the MWRI observations over ocean, by using GDAS data as model inputs. “O-B” Results and “Double difference” results show that: 1). the on-orbit calibration status of MWRI is stable, 2). the brightness temperatures from MWRI observation are highly consistent with those derived from AMSR-E and model simulation. Precipitation products as well as surface soil moisture and snow depth products derived from MWRI L1 data also were also introduced in this paper.