The effect of the phosphate anion on the stabilization of hydrate in magnesium oxychloride cement(MOC) was investigated by the method of XRD. The results indicate that the addition of phosphoric acid to magnesium oxychloride cement pastes can greatly improve the stability of hydration products in the water. It is concluded that the improvement of the stability could be attributed to the coordination between the phosphate anion produced by ionization of phosphoric acid and Mg 2+ ion, which makes the lowest concentration of Mg 2+ ion required by the stable existence and the formation of the MOC hydrate be greatly decreased.