Abstract: In order to grasp the compression characteristics and microscopic mechanism of calcareous sand in the South China Sea, we did compression tests on three different groups of calcareous sand(S1:1.43~2mm, S2:0.5~1mm, S3:0.5~2mm). We used a self-made sand microstructure extraction device to obtain the microstructure of calcareous during compression. The results indicate that:(1)The size, shape and grade of the calcareous sands significantly affect the fracture of the particles. When the pressure is lower than 800kPa, the S1 group with large particle size is mainly broken by the angular break of the sand particle; but the S2 group with smaller particle size does not have obvious cracks. That's means relatively regular particle morphology leads S2 group to compress mainly by rolling and redistribution of the particles. For the well-graded S3 group, particles have no obvious cracks except for some low-width broken limb-shaped ones. (2)When the pressure is larger than 800kPa, S1 group of calcareous sand turns to the broken form dominated by the overall destruction of the particles; S2, S3 with the increase of the compactness, the damage of the two groups of calcareous sand sample is dominated by the overall fragmentation of the particles. Based on the microstructure changes of these different groups of calcareous sands during the compression, we summarized four contact modes which control the crushing of calcareous sands: point-line contact, line-surface contact, surface-surface contact and composite contact. These contact models can be used to determine the form of particle breakage under different conditions. Finally, we discussed the effect of crushing process on the geometry parameters of calcareous sands.