Taking the Northern Hemisphere stratospheric sudden warming(SSW) event in January 2021, which may have an important impact on the winter cold wave event in China, as an example, this study discusses the characteristics and mechanism of stratospheric sudden warming in January 2021 by using observation data, reanalysis data and prediction model. During the occurrence of this SSW event (hereinafter referred to as "event")，the Arctic stratospheric temperature rose by nearly 30 K in a few days, and the westerly winds weakened over the Arctic, and even reversed to the easterly winds three times in this event. In the early stage of the event, the Pacific Sea Surface Temperature (SST) and the Barents Kara Sea (BK) sea ice decreased abnormally; Through studying the anomalies of Eliassen-Palm (E-P flux) study, it is found that this leads to more tropospheric planetary wave propagate into the stratosphere; The increasing of planetary wave activity in the stratosphere caused by SST and sea ice may be the main factor to the SSW event. In addition, it is also found that the positive anomaly of stratospheric geopotential height propagated downward into the troposphere during the event, which is significantly related to the extreme cold event on the Eurasian continent in mid-January 2021. Finally, the stratospheric prediction model based on WACCM6 model and DART assimilation tool is used to study this event. The model successfully predicted the SSW event in January 2021. The predicted temperature and zonal wind intensity are basically consistent with the observation results. The results further show that assimilating stratospheric temperature, ozone and water vapor play an important role in accurately predicting the SSW event via reducing the errors of stratospheric circulation and temperature in the initial prediction field. If not assimilating stratospheric temperature, ozone and water vapor in the initial field, the results are very different from the observation, or even completely opposite.