Abstract: The upgrading and reinforcement of existing embankments in hydraulic engineering projects demonstrate significant economic benefits. This study, based on the Huayang River Flood Storage and Retention Basin Construction Project, utilizes field-measured data from the Jiucheng section embankment filling process and numerical simulation back-analysis to investigate the deformation patterns and underlying mechanisms of existing levees and their underlying deep soft foundations under new embankment filling. The main conclusions are as follows: 1. The settlement of the embankment increases with the filling height, while the settlement rate gradually decreases over time. 2. The filling of the new embankment complicates the stress history of the underlying deep soft foundation, with the maximum differential settlement occurring near the toe of the new embankment. 3. Back-analysis through numerical simulation validated the influence of the additional load from the existing levee on the consolidation effect of the underlying soft foundation. Employing a radial zoning method in numerical modeling that considers the additional load effectively simplifies the representation of the complex stress history in deep soft foundations.