Abstract: Sandwich composites have shown great potential in the design of lightweight structures for deep-sea equipment due to their excellent specific stiffness and specific strength. However, systematic experimental and numerical studies on composite sandwich shells, particularly regarding the ultimate load-bearing mechanisms and damage evolution laws of pure carbon fiber and carbon fiber-foam-carbon fiber sandwich configurations under the equal weight-to-displacement ratio condition, remain insufficient. Therefore, this study prepares pure carbon fiber and carbon fiber-foam-carbon fiber sandwich thin-walled shells with equal weight-to-displacement ratios, conducts four-point compression tests, and further embeds a Fortran-written three-dimensional Hashin failure damage subroutine in Abaqus and combines it with cohesive elements to conduct numerical simulations of these two types of shell models. The test results show that under the equal weight-to-displacement ratio condition, the average peak load of the carbon fiber-foam-carbon fiber sandwich shells is approximately 24.7% higher than that of the pure carbon fiber shells, and the stiffness is increased by about 151.9%. This significantly improves the load-bearing capacity and deformation resistance of the shell structure. From the failure morphology and strain response, it can be seen that the failure of the pure carbon fiber shell is caused by interlaminar delamination of the carbon fiber layers, while the failure of the sandwich shells is mainly due to the fracture of the foam core material dominated by the maximum principal stress. Moreover, as the strength of the core material gradually increases, the damage mode of the sandwich shell will shift from the brittle fracture of the foam core material to the failure of the carbon fiber composite layers. The load-displacement curves, circumferential strain distributions, and damage morphologies obtained through the numerical simulations of both the pure carbon fiber shells and sandwich shells show good agreement with the experimental results, validating the effectiveness of the combined approach using the three-dimensional Hashin failure criterion and the cohesive model for predicting the failure of carbon fiber composite shells. This work provides important support for the subsequent design optimization, manufacturing, and application of high-pressure-resistant carbon fiber composite shells in the deep-sea environment.