Solid buoyancy material is an important material for manned/unmanned deep submersibles due to its low density and high compressive strength. Hollow glass microspheres in solid buoyancy materials will gradually damage at a high pressure, leading to an increase in water absorption and thereby reducing the positive buoyancy of the submersible and affecting the safety and usability of manned/unmanned deep submersibles. There is a coupling relationship between water absorption rate and stress of buoyancy materials, which poses challenges in calculating the water absorption rate of buoyancy materials. This paper was to investigate concentric spherical shell cells and analyze the entire water absorption process via adding subroutines by a finite element method (FEM). The distribution, changes, and interrelationships of bulk stress, water absorption rate, and diffusion coefficient of buoyancy materials at different thickness points and durations were analyzed. It is indicated that water absorption rate is linearly related to bulk stress, and the expansion coefficient is related to bulk stress. The results calculated by the finite element analysis are in reasonable agreement with the experimental data. The paper can provide a theoretical guidance for the design and application of solid buoyancy materials at sea depths.