Spaceborne tomographic synthetic aperture radar (SAR) (TomoSAR) uses stacks of SAR images acquired at slightly different positions over a certain period in a repeat–pass manner, to obtain absolute 3D positions and 4D deformations of a large amount of building scatters. The double-tier permanent scatterer (PS) network construction method without pre-removal of the atmospheric phase screen has been successfully applied in TomoSAR, but it suffers from the problem that the PS network cannot cover the whole study area when applied to a larger range of building complexes. To address this problem, this paper proposes a new method of double-tier PS network construction combined with a region-growing algorithm, which introduces a region-growing algorithm in the middle of the first-tier network and the second-tier network to realize the construction of a PS network that covers the entire study area after several loop iterations. Experiments are carried out using 26 scenes of TerraSAR-X/TanDEM-X 3-m data stack in Nanjing, China, and the experimental results show that the proposed method improves the PS coverage in the first-tier network from 72.5% to 99.6%, and the network expansion efficiency value increased from 0.12 to 21.48. Its effectiveness and robustness are verified by comparison with other methods.