The foundations of islands in the South China Sea are constructed by hydraulic filling method. There are widely discontinuous gradation states in which large and small particles coexist in the hydraulic fill formation of island. The discontinuous conditions of particle gradation have a significant influence on the mechanical properties of the foundation material of coral sand. In this study, a series of drained triaxial shear tests under different effective confining pressures, relative densities, and coral gravel contents were carried out on coral sand-gravel mixtures, to investigate the strength and deformation characteristics of the gap-graded coral sand under static load. Also, the feature of particle breakage in the tested gap-graded soil has been studied. The results show that the stress-strain relationship curve of the coral sand sample is significantly affected by the confining pressure and relative density. The peak friction angle and dilatancy angle of coral sand-gravel mixtures decrease with the increase of the effective confining pressure, and increase with the increase of relative density. Under the same effective confining pressure, the strength of coral sand-gravel mixtures at different relative densities exhibited different growing patterns with increasing gravel content, which may be related to the contact state and composition ratio of the coarse and fine particles in coral sand-gravel mixtures. In addition, this study explored the correlation between the plastic work with relative breakage index for different gravel contents of coral sand-gravel mixtures and established the correlation function between the relative breakage index B_r and the plastic work W_p on this basis.