The foundations of islands and reefs are constructed by hydraulic filling method. The coral layers are often at discontinuous gradation states in which large and small particles coexist. The gap-graded condition has a significant impact on the mechanical properties of the hydraulic fill coral sand. In this study, a series of drained triaxial shear tests under different effective confining pressures, relative densities, and coral gravel content were carried out on coral sand-gravel mixtures, and the strength and deformation characteristics of coral sand-gravel mixtures under static loading were investigated. This study also analyzed the evolution law of particle breakage for coral sand-gravel mixtures. The results show that the stress-strain relationship curve of the 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 an increase in effective confining pressure, and increase with level of relative density. Under the same effective confining pressure, the strength of coral sand-gravel mixtures at different relative densities exhibited different varying 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 content of coral sand-gravel mixtures and established the correlation function between the relative breakage index Br and the plastic work Wp.