Abstract: Coral sand, as a special bioclastic sediment, is widely distributed in the South China Sea. It has many engineering characteristics such as porous, heterogeneous and discontinuous. As the basis of island filling materials and engineering construction, the mechanical properties of the dredged coral sand are the key scientific issues in the research of coral island and reef engineering. The South China Sea has a large amount of rainfall. The dredged coral sand has good permeability and high engineering bearing capacity. The influence of water content and high load on the shear mechanical properties of the sand is very significant. Based on the analysis of gradation and composition, the conditions of different compactness and water content were studied by using a self-developed large-scale shear mechanical testing machine for soil rock mixture and an island reef in Nansha, China. The results show that:(1)The increase of water content leads to the decrease of shear strength, which has a negative correlation; the increase of compactness leads to the increase of shear strength, which has a positive correlation; (2)The influence of water content on the angle of internal friction is significant, showing a negative correlation. When the water content is greater than 10%, the angle of internal friction decreases slightly; when the density is 90%, the water content is greater than 5%, and the cohesion decreases slightly; (3)The influence of the compactness on the cohesion of the coral sand is not obvious, and the influence of the compactness on the angle of internal friction is significant, when the water content is greater than 5%, with the increase of the compactness internal friction angle increases significantly; (4)Under the high load, the influence of water content and compactness on the shear mechanical properties of sand is significant. The sand with small water content and large compactness has the strongest shear properties, which is of guiding significance for the design of the island filling project and the modification of site conditions.