Abstract: To study the effect of calcium ion and pH on phosphorus content in the water in the Yangtze Estuary Reservoir, the rule of phosphorus adsorption and release in the sediment and the change of various forms of phosphorus affected by calcium ion and pH was analyzed, and the components of the sediment was further analyzed by XRD technology. The result suggested that Ca²⁺ significantly increased the adsorption of sediment on phosphorus while inhibiting the release of phosphorus from sediment, and thus the content of phosphorus in the water decreased. With the action of Ca²⁺, under pH<3, the sediment significantly decreased the release of phosphorus, it increased the content phosphorus in the water. Under pH>3, the sediment significantly increased the adsorption of phosphorus, and decreased the content phosphorus in the water. When the content of Ca binding P (Ca-P) fraction increased in the sediment, the content of Ca₅(OH)(PO₄)₃increased, and new material of Ca -binding crystalline compound appeared on the sediment. In alkaline condition, the content of phosphorus in the water decreased because of Ca²⁺.