Tianjin is one of the inland cities with the most severe cases of subsidence hazard in China. The majority of the existing studies have mainly focused on Beijing-Tianjin-Hebei, and little attention has been given to Tianjin. In addition, these existing studies are short-term investigations, lacking long-term monitoring of surface subsidence. In the present study, we use the Small Baseline Subset Interferometric Synthetic Aperture Radar ( SBAS-InSAR) technique to monitor the subsidence process in Tianjin between 2015 and 2020 and reveal its spatial and temporal variation. We divided the 44-view Sentinel-1A image data into three periods to avoid the effect of temporal and spatial decoherence by extracting the surface deformation field in Tianjin. We finally verified the accuracy and reliability of the inversion results using second-order leveling data. Results showed that the correlation coefficient r between the two reached 0.89, and the root mean square error was 4.84 mm/y. Obvious subsidence funnels exist in Tianjin, mainly in the towns of Wangqingtuo and Shengfang. These subsidence funnels have a subsidence deformation rate of-136.2 mm/y and a maximum cumulative settlement of -346.3 mm within the study period. The subsidence area tends to extend to the southwest. The analysis of annual rainfall, groundwater resource extraction, spatial location distribution of industrial areas combined with SBAS-InSAR inversion results indicates that overextraction of groundwater resources is the main cause of land subsidence in the area. Therefore, strict control of groundwater extraction is the main approach to mitigate land subsidence effectively.