Multi-channel scan radiometer(MCSR)on FY-2C Satellite launched in October,2004 has been working in orbit for two years. In order to improve quantitative application and track long-term changes in the visible channel,an onboard calibrator using a small beam of sunlight as source is designed,which is fixed on the second mirror and made up of two plates,two prisms and a set of baffles. The calibrator including optical structures,ground-based tests and on-orbit parameters are concisely described. In pre-launch experiments a mirror is used to lead the sunbeam into the MCSR,but in-flight performances the instrument can get the sun signal every midnight. By analyzing and comparing the signals and images,differences between them can be corrected,channel performances such as attenuation can be acquired in time and data also can contribute to better calibration results.