Sand island or cay is the dominant type of coral reef islands in the South China Sea. This paper gives in detail the sand islands composed of loose accumulational materials on the atoll reef flats in the Xisha Islands, their types, characteristics of morphological structure and evolutional pattern and dynamics. Sand island or cay is the dominant type of coral reef islands in the South China Sea. This paper gives in detail the sand islands composed of loose accumulational materials on the atoll reef flats in the Xisha Islands, their types, characteristics of morphological structure and evolutional pattern and dynamics.