Abstract: Based on the calcareous sand compaction experiments. Using glue after making the compaction of calcareous sand undisturbed samples. Using SEM to scan the sample and discuss the sample mesoscopic structure and the relationship between dry density. Results showed that the calcareous sand compaction curve can be divided into initial dry density drop zone, dry density increase and the decrease of the dry density of the secondary area three stages. Through the SEM images can be seen: The compaction of calcareous sand sample more than large particles formed after the skeletal structure, contact between particles of point-edge; the contact relationship between small particles for side-the side. The changes of the calcareous sand dry density of the compaction is the process of the changing process of pore between particles. Find out the control of the grain group was 0.1～0.25mm.