Abstract:   Objectives:   A major problem in Beijing is land subsidence caused by long-term over-exploitation of groundwater. Since the opening of the south-to-North Water Diversion Project, the problem of water shortage in Beijing has been greatly alleviated. The south-to-north Water Diversion Project alleviates the land subsidence in Beijing to a certain extent.   Methods:   In order to analyze the development trend of land subsidence after the start of the South to North Water Transfer in the Beijing, ascending and descending time series interferometric synthetic aperture radar(InSAR) technique is used to monitor land subsidence in Beijing. Firstly, the mean deformation velocity and cumulative deformation in line of sight (LOS) in Beijing district from January 2015 to December 2020 obtained by the small baseline subset InSAR(SBAS-InSAR). Secondly, The Robust least square fitting method were used to fuse the deformation results of the lifting rail and compare the global positioning system (GPS) monitoring datas and the fusion results of lifting rail. Finally, analyze the variation trend between the deformation results by the Robust least quadratic fitting and groundwater.   Results:   The deformation results show that the center of Beijing is basically stable and the deformation distribution is not uniform. The maximum ascending annual deformation velocity and the maximum ascending cumulative deformation amount reach -134mm/year and -697mm, the maximum descending annual deformation velocity and the maximum descending cumulative deformation amount reach -135mm/year and -734mm.And the fusion results obtained by the least square fitting method have certain reliability and accuracy.   Conclusions:   The subsidence rate in Beijing district shows a decreasing trend with the gradual increase of groundwater level. In general, the middle route of South-to-North Water Transfer Project has greatly alleviated the expansion trend of land subsidence in Beijing to a certain extent.