Abstract: According to the laws of simultaneous equilibria and mass conservation, the logarithm concentration-pH diagram of a Ca-Mo-P-H₂O system, the logarithm eoncentration-pH diagrams of total calcium and total molybdenum at different total phosphate concentrations, and the stabile area of calcium molybdate at 25℃ were obtained on the basis of thermodynamic data. The results show that the stabile areas of H₂MoO₄(s), CaMoO₄(s), Ca₅(OH) (PO₄)₃(s) and Ca(OH)₂(s) are distinguished orderly with increasing pH values at a constant total phosphate concentration. Calcium molybdate can be easily decomposed since the stabile area of calcium molybdate shrinks or even disappears with increasing total phosphate concentration; at the same time, the total molybdenum concentration increases, and the total calcium concentration decreases. However, high phosphate concentration is unnecessary for calcium molybdate can be decomposed at a very low phosphate concentration. Compared to carbonate, phosphate is more propitious to the decomposition of calcium molybdate.