Persistent Scatterer Interferometry (PSI) is a multi-epoch InSAR method for measuring displacements of Earth's surface over time by exploring the interferometric phase of constantly coherent points, named Persistent Scatterer (PS). PSI has shown its superior capability to monitor linear features with long-distance and large-span characteristics. Nevertheless, the interpretation of estimated displacements over PS is a challenge to overcome. Here, we propose a target characterization approach for multi-epoch InSAR post-processing to efficiently correlate the target's possible deformation types and corresponding deformation characteristics. The PS targets were categorized into elevated (ELV) and ground (GRD) groups. The deformation signals are assigned into four types: Defo-Type I stable, Defo-Type II shallow, Defo-Type III structural, and Defo-Type IV mixed deformation based on the ELV versus GRD differential motion estimation. Our approach is demonstrated over the metro network of Shanghai city, China using High-resolution TerraSAR-X data. The seven-year displacements map along Shanghai metro lines estimated by PSI shows that 85% of the points are relatively stable, while only 0.07% exhibits a significant deformation trend. We performed the stability evaluation along the subway by incorporating velocity, cumulative and relative deformation. It was found that the lines that have been operated for a long time are relatively stable, and obvious deformations were observed on the newly built lines and the extension sections of existing lines. By interpreting deformation types, We inferred that the displacements along metro lines mainly corresponded to Defo-Type II, and the areas with obvious deformation involve the superposition of multiple deformation types.