Phosphorite, or guano, or insular phosphorite, commonly containing 10-35% P_2O_5, is one of the sources of phosphatic fertilizer. Taking the modern insular phosphorite on the Xisha Islands as an example. this paper discusses the ore-forming mechanism of phosphate and the important contribution made by animals to this process.The phosphorite occurs in the middle part of carbonate sand cays of coral reef surrounded by longshore sandbanks, which assumes a dish-shape. The surface of the cays is covered by unconsolidated guano. The phosphorite, yellowish brown to dark brown, is made up of thick-bedded sand and gravel cemented by collophanite, and partly replaced by carbonate apatite.The mechanism of mineralization is as follows:The average content of phosphorus in the sea water is only 0.07 ppm, but through the sea food chain concentration, phosphorus content may become higher. The phytoplanktons, as producers, suck up phosphorus out of sea water. The primary consumers, zooplanktons, obtain phosphorus by eating phytoplanktons, then they are followed by the second and third consumers. Sea birds living on fish, as the third consumers, concentrate and transport the phosphorus to the carbonate sand cays. Finally layers of guano are deposited under the arbores.Under the tropical climatic conditions with high temperature and plentiful rainfall soluble matters are leached out of the guano and infiltrated into deeper horizon. In alkaline groundwater with pH greater than 7.0. the phosphoritization takes place, bioclastics are cemented and replaced by collophanite or carbonate apatite. Finally, the phosphorus is fixed and phosphate formed.