During 2-4 March 2021, a continuous low-cloud process in Beijing caused large deviation of temperature forecast. Both the global/regional numerical models and the forecasters failed to predict this process. Using the conventional meteorological data, ERA5 reanalysis data and FY-4A high-resolution visible cloud images, combined with the data of ceilometer and cloud radar, this paper discusses the formation and maintenance mechanism of the low cloud. The results are as follows. The favorable background for the formation of the low cloud was that there was no influence of obvious cold air after precipitation and the ground humidity was not well removed in the boundary layer. Meantime, the low cloud developed and maintained with stable atmospheric stratification, weak ascending motion and topographic effect. The warm advection and the growth of wind speed at 925 hPa destroyed the stable stratification, leading to the enhancement of mixing activity in the boundary layer and thus the dissipation of the low clouds. Detailed information of the cloud base height and cloud structure can be obtained via the ceilometer and cloud radar, which can be served as a useful complement to conventional observation.