The coral sand with coarse and fine sized particles forms a discontinuous graded coral sand strata under hydraulic sorting. Under complex ocean dynamic conditions, fine particles are prone to loss leading to internal erosion, posing significant challenges to the stability of island and reef foundations. To further investigate the internal erosion mechanism of gap graded coral sand, high-pressure oedometer tests were conducted on three types of gap-graded coral sand samples. The compression coefficient of gap-graded coral sand shows a trend of first increasing and then decreasing with the increase of fine particle content. The relative breakage of gap-graded coral sand decreases linearly with the increase of fine particles. In the mixed sample, when the content of fine particles is low, fine particles mainly fill the voids between coarse particles, and the particle crushing is mainly dominated by coarse particles. The compression coefficient is affected by the ratio of coarse and fine particle. When the fine particle content exceeds the transition fine particle content of 50 %, the coarse particles are surrounded by fine particles, and the coarse particles are almost not broken, and the compression coefficient is almost not affected by the ratio of coarse and fine particle. The curves of coral sand with different fine particle contents can be represented by a hyperbolic model, and the model parameter b is the reciprocal of the initial porosity ratio, and the model parameter a is related to particle breakage. The compression characteristics of gap-graded coral sand under extremely high stress were reasonably predicted by this model. Under extremely high stress, the smaller the ratio of coarse to fine particles, the better the convergence of the compression curve. This study can provide theoretical guidance for the construction of island and reef infrastructure, and has certain academic value and broad engineering application prospects.