In this paper, ground-based radar and spaceborne Interferometric Synthetic Aperture Radar (InSAR) images were combined to monitor slope stability and analyze the main deformation factors of an ancient landslide on the right bank of the Dajinchuan River in Danba County, Sichuan Province, China. We applied the short baseline set (SBAS) time series strategy with 656 scenes of ground-based radar between September 13-17, 2019, and 62 scenes of Sentinel-1 data from July 2018 to October 2020. Combined with high-resolution satellite images and digital elevation model (DEM) data, we acquired trace and quantitative deformation features and discussed the factors that contributed to slope instability, such as geological structure, topography, external environment and human activities. The largest deformation area detected by ground-based radar is located in the bedrock above the target area with a maximum cumulative deformation of more than 30 mm during the detection time. The maximum average annual deformation rate detected over the region by spaceborne InSAR is over 40 mm/a. We analyzed the differences between the ground-based radar and spaceborne InSAR and the reasons for the differences. This study provides references and suggestions for investigating potential landslide risks by combining ground-based radar and spaceborne InSAR technology.