To address problem of calcareous sand encountered at the backfill project at the island reef in the South China Sea, in this work, an eco-friendly agent, i.e., polyurethane foam adhesive (PFA), was proposed to improve the ground associated with calcareous sand. Evolution of shear strength, cohesion and internal friction of PFA-improved calcareous sand with various particle size distribution and PFA content, as well as the stabilization mechanism were investigated by performing a series of direct shear tests and microstructural analysis using scanning electron microscope (SEM). The test results show that, as the PFA content increases, the shear strength of calcareous sand significantly increases and the shear stress-displacement curve gradually transits from hardening to softening behavior, where the shear stress increases to a peak value following by a subsequent reduction with the increase of shear displacement. The increase of PFA content would result in a significant increase in the cohesion of improved calcareous sand, in contrast, it has limited influence on the change of internal friction angle of the specimen. The higher content of coarse calcareous sand particles in the specimens, the higher effectiveness of using PFA in improving the shear strength of calcareous sands due to its larger inter-granular pore space where the PFA could be readily filled in with active chemical reaction. The SEM results also reveal that the polyurethane foam adhesive could be effectively coated on the surface of the calcareous sand particles and filled among its void space, thereby significantly improving the shear strength of the calcareous sands.