<p>Underwater vacuum preloading has shown potential for improving soft marine soils in specific applications, particularly in intertidal zone projects. However, its widespread implementation remains constrained by limitations in current underwater sealing technologies. This study evaluates the efficacy of a proposed mud-sealing method using mud with low permeability to replace the conventional sealing technology with a plastic membrane. An indoor model test was conducted to evaluate the effectiveness of the underwater vacuum preloading sealed with mud (UVPSM) as well as the behaviors of the foundation soil through real-time monitoring of pore pressure, vacuum transfer efficiency, consolidation settlement, and volumetric change in the covering water. The change in water content and undrained strength was also tested to investigate the improvement efficacy. Moreover, finite element (FE) simulation was performed based on the experimental results to further investigate the behaviors of the sealing mud layer and the foundation soil. The impact of the covering water was also discussed based on the simulation. Additionally, practical recommendations were provided for the application of underwater vacuum preloading using the mud-sealing method.</p>