The microwave scatterometer onboard HY-2A ocean satellite primarily measures surface wind speed and direction over the global oceans. A precise in-orbit radiometric calibration of the scatterometer is crucial to accurately measure both wind speed and direction. This study is the first to perform the in-orbit radiometric calibration of the seawinds scatterometer on the QuikSCAT spacecraft by using global open ocean backscatter measurements. The ocean calibration method is analyzed and discussed on the basis of the result of weawinds that was precisely calibrated. Results show that the accuracy of the ocean calibration method is a pproximately 0. 15 dB for a specific Geographic Model Function（ GMF）. The deviation among different GMFs in ocean calibration is caused by systematic bias. The GMF must be accurate enough to realize the absolute calibration of the scatterometer. The calibration method and results from seawinds are used as a basis to implement the radiometric calibration of the HY-2A microwave scatterometer. Our results suggest that the calibration coefficients for the HY-2A microwave scatterometer are- 1. 79 dB for VV polarization and- 1. 73 dB for HH polarization. The wind vectors of the HY-2A microwave scatterometer are also validated by wind observations from NDBC ocean buoys. The accuracy of retrieving wind speed and direction is significantly improved after applying the obtained calibration coefficients.