Ammonium polyphosphate is widely used in water soluble fertilizer industry due to its excellent water solubility, slow-release property and chelation. However, ammonium polyphosphate is easily hydrolyzed into ammonium dihydrogen phosphate and diammonium hydrogen phosphate in storage, and the composition of its aqueous solution changes continuously. Its solubility decreases with decreasing polymerization rate, causing crystallization and precipitation in the storage solution, that increases the cost of transportation and application. The quaternary phase diagram of ammonium dihydrogen phosphatediammonium hydrogen phosphateammonium polyphosphatewater at 25℃ was established by using Schreinemaker wet slag method, and the mass distribution of each component in saturated liquid and wet slag phase was obtained by ion chromatography and mass conservation equation. The reasons of crystallization and precipitation of ammonium polyphosphate solution were explained, which was a guidance for storage and use of ammonium polyphosphate as a high efficiency water-soluble fertilizer. The polymerization degree of ammonium polyphosphate was diverse due to the variety of ammonium polyphosphate products produced by the manufacturers. There were different solution systems while the ammonium polyphosphate as a compound fertilizer raw materials, and it was time-consuming and laborious to obtain phase diagrams by experiments for each system. The calculation of watersalt phase diagram by mathematical model can save a lot of experimental work and provide the basis for the agricultural use of ammonium polyphosphate. In order to quickly obtain phase diagrams of other ammonium polyphosphate systems, local composition model of electrolyte solution was introduced to get solvent (salt)salt interaction energy parameters of the experimental system. The phase diagram calculated by regression parameters was in good agreement with the experiment, and it can be used to predict the phase diagram of other ammonium polyphosphate systems.