Abstract: This study investigates the deformation mechanisms and internal particle migration characteristics of geotextile sand containers (GSCs) under tropical high-humidity conditions. Laboratory experiments were conducted involving simulated rainfall to monitor external deformation, internal pore water pressure, and particle gradation changes. The results demonstrate that the permeability coefficient of the Hainan Bay sea Sand (sourced from the Qiongzhou Strait) significantly influences the timing of peak pore water pressure development and the initiation of suction. Particle size stratification was observed within the GSCs during rainfall simulation; however, this stratification was not distinctly evident in the upper, middle, and lower sections of specimen S-A(B/C)-15. Furthermore, the lower layer of sea sand in specimen S-C-2(5/15) transitioned from a poorly graded (SP) to a well-graded (SW) classification. Under wet-dry cycles, the mass loss patterns of the GSCs were quantified, revealing that the relationship between natural drainage time and the drying process for specimen S-A(B/C)-15 at room temperature was well-described by an exponential function. In the absence of external load, GSC deformation exhibited a linear correlation with rainfall duration, characterized by slow, stable progression and a settlement rate lower than that observed in field applications. These findings provide critical insights for the engineering design of GSC dimensions and inform the application of Hainan Bay sea Sand in civil engineering projects within tropical, high-humidity environments.