Calcareous sand is the only building material in the reclaimed foundation of island-reefs in the South China Sea, and calcareous sand in reclaimed foundation is predominantly calcareous gravelly sand (CGS). Particle breakage is the primary factor affecting the stability of reclaimed foundation. Considering the susceptibility to breakage of calcareous sand, multiple triaxial shear tests are conducted to investigate the particle breakage and evolution of CGS under different relative densities and effective confining pressures. Test results show that relative breakage rate more satisfactorily reflects the overall particle breakage degree of sample rather than grading parameters and fractal dimension. Thus, the relationship of relative breakage rate with relative density and effective confining pressure is established. A trend of “contraction first and dilatancy afterwards” is observed for CGS, which makes plastic work done by mean effective stress exist the negative value. Besides, a plastic work calculation model is developed to reflect the effects of relative density and effective confining pressure. Moreover, the relationship between plastic work and relative breakage rate can be fit using an inverse proportional function and is not significantly affected by relative density or effective confining pressure. These findings in this study lay a theoretical foundation for assessing the stability of reclaimed foundation of island-reefs.