Abstract: The sea surface height (SSH) calibration of spaceborne radar altimeters is crucial for ensuring their measurement accuracy and expanding the high-precision application of data. As one of China's HY-2 series satellites, the HY-2D satellite is equipped with a dual-frequency radar altimeter, but no SSH calibration research has been published since its launch, because the Wanshan calibration field is far from the sub-satellite point. This study realized the absolute SSH calibration of the HY-2D satellite radar altimeter using the observation data of a multi-antenna GNSS buoy deployed in the Qianliyan Islet calibration field from May to June 2023, combined with the data from the Qianliyan tide gauge well and seabed-based moored pressure gauge. Firstly, the observation data of the multi-antenna GNSS buoy were processed by dynamic differential positioning, with a focus on correcting the seawater density error and the non-zero mean attitude error. Secondly, the ranging data from the HY-2D satellite SGDR product were extracted, and another set of ranging data was obtained using the adaptive waveform retracking method. The altimeter SSH was calculated by combining various error correction items. Finally, through three corrections including mean sea surface, ocean tide, and reference ellipsoid, the comparison between the buoy SSH and the altimeter SSH was realized, and the absolute calibration was completed. The results show that the multi-antenna GNSS buoy can achieve long-term stable observation at sea, and the SSH measurement accuracy can be effectively improved after attitude error correction. Using the combined scheme of "adaptive waveform retracking + corrected radiometer tropospheric wet delay", the mean calibration bias is close to 0, the standard deviation is 2.8 cm, and the calibration accuracy reaches 1.2 cm. The correction effect of the corrected radiometer on tropospheric wet delay is better than that of the ECMWF model, which significantly improves the calibration accuracy. The calibration results of this study can provide a reliable evaluation for the data product quality of the HY-2D satellite radar altimeter, and the multi-antenna GNSS buoy used provides technical support for the subsequent calibration of wide-swath imaging altimeters.