Abstract: The scale of reef engineering in the South China Sea is gradually increasing. The construction of some structures with large loads can inevitably lead to the crushing of calcareous sand that can be the main material of foundation. Therefore,it is important to study the compression and the crushing mechanism of calcareous sand under high stress. In this paper,the basic geometric parameters of calcareous particles were obtained based on the microscopic image acquisition and processing technology. The shape of particles was quantitatively characterized by two shape parameters of sphericity(S) and convexity(C). A series of confined compression tests,based on a high-pressure oedometer instrument with a termination pressure of 16 MPa,was carried out to investigate the compression and breakage characteristics of calcareous sand in high stress. The effect of particle size,particle distribution,and particle shape on the compressive and crushing properties of calcareous sand were also discussed respectively. The result showed that with the increase of average particle size,the shape of calcareous sands was more irregular,and the corners were also developed. With the increase of stress,the compression curves of calcareous sand with different particle sizes tend to converge on a straight line. The influence of initial particle size on the compression characteristics of the samples decreased and disappeared eventually. Meanwhile,for calcareous sand of different grades,the compression curve also converged but did not intersect in a straight line. The compression characteristics of samples were still influenced by the initial grades. It's worth noting that when the non-uniformity coefficient(C_u) wass close,the compression and a crushing amount of calcareous sand gradually increased with the increase of average particle size(d₅₀). However,when d₅₀ was close,its compression and crushing amount gradually decreased with the increase of C_u.