Abstract: Shanghai has been undergoing over a century of land subsidence, which is increasingly complex in terms of its mechanisms and causes in recent years. This study uses long-term settlement data extracted from layered extensometers located in the three monitoring stations on Chongming three islands. This study characterizes deep and shallow soil layer settlement under minimal subsidence conditions. Results show that shallow soil layers settle continuously, while deep layers remain stable or even rebound. Shallow soil layers contribute over 89% to land subsidence, making them the primary contributor. Analysis of shallow soil properties and external drivers reveals the primary causes of shallow soil settlement in Chongming three islands:①Rainfall changes alter phreatic water levels, triggering elastic deformation in shallow sand layers.②Foundation pit construction activities trigger consolidation settlement in clay layers. This creates plastic deformation within compressible soil strata.③Land reclamation projects cause seepage consolidation in soft soils, generating secondary consolidation that decreases over time.These findings reveal new subsidence features in Chongming three islands and clarify shallow layer mechanisms, providing critical reference for developing precise ground subsidence control strategies.