There are many cities in China suffering seriously from land subsidence caused by groundwater over-extraction, but sufficient money has been lacking in monitoring the subsidence. Compared with leveling and GPS surveying, D-InSAR is more precise and cost-efficient. Tianjin, a city with smooth topography and severe subsidence, is an ideal place for testing and analyzing D-InSAR technique, especially with plenty of Envisat and ERS-1/2 SAR data and thousands of valuable leveling data being available. However, D-InSAR is liable to be contaminated by atmosphere delay, temporal decorrelation and baseline errors. These errors cannot be removed by SAR data processing. This paper analyzes the features of atmosphere delay with the auxiliary data in Tianjin urban area. The results demonstrate that D-InSAR can detect subsidence within three months. Further actions have been undertaken to improve the D-InSAR subsidence monitoring system, such as using Envisat data, gathering GPS zenith delay data, etc. to reduce errors. The final interest of our research is to establish a robust, cost efficient city subsidence monitoring system by using D-InSAR technique.