In order to accurately separate the contributions of foundation settlement and structural damage to the deformation of port approach bridges and realize the physical attribution of structural damage， an integrated framework of "multi-source perception–physical modeling–deviation diagnosis" was adopted to develop a synergistic method combining time-series PS-InSAR-based foundation settlement monitoring， high-resolution optical image shadow analysis， and BIM-based parametric mechanical modeling. First， under a unified spatiotemporal datum， time-series PS-InSAR technology was applied to extract the foundation settlement field， while an improved Normalized Shadow Index （NSI） was used to invert the relative deformation at the tops of bridge piers. Second， an LOD350-level BIM model was converted into a parametric beam-grid mechanical model， and foundation settlement as well as thermal loads were taken as inputs to calculate the theoretical deformation response. Finally， a Damage Risk Index （DRI） was constructed to quantify the deviation between monitored and theoretical deformations， enabling damage early warning and localization. Closed-loop verification was further performed using an actual engineering case. The results showed that the proposed method achieved a mean absolute error （MAE） of approximately 1.14 mm and a root mean square error （RMSE） of approximately 1.46 mm in deformation monitoring， with 88% of data points having an error no greater than 2 mm and a damage identification accuracy of 93.7%. This method also supports the full-chain diagnostic process of "large-scale early warning – localized positioning – on-site verification – repair validation". It is concluded that this method effectively overcomes the limitations of single remote sensing techniques in interpreting deformation causes， achieves the transition from "phenomenon perception" to "mechanism interpretation"， and thus provides a reliable technical paradigm for the intelligent operation and maintenance of long linear steel-structure infrastructures such as port approach bridges.