In this paper, the method for FY-3A/MWHS calibration in-orbit has been introduced. Radiometric calibration of FY-3A/MWHS on orbit has been performed based on the T/V test results and surface lab performances test results. It is confirmed that the system of radiometric calibration for MWHS is successful, and performs well on orbit. The variations among space views and warm target views in the interval of scan lines are generally within 20 counts. And the temperature fluctuation of warm target is within 0.5 K in a single pass. Due to the accuracy of the nonlinearity parameters, the radiance calibration of MWHS agrees well with NOAA-17/AMSU-B. The brightness temperature differences between FY-3 MWHS and NOAA-17/AMSU-B at simultaneous cross overpass points are less than 1.5 K. That is the foundation of quantitative application for the data of FY-3A/MWHS in numerical weather prediction(NWP). The method of calibration and validation for FY-3A/MWHS will be a good demo for the other passive microwave sounder in the future.