利用Schreinemaker湿渣法，通过离子色谱和质量守恒方程联立得到各组分在饱和液相和湿渣相中的分布，从而得到25℃下磷酸二氢铵磷酸氢二铵聚磷酸铵水四元空间相图，解释了聚磷酸铵水溶液结晶、沉淀的问题，对聚磷酸铵作为高效水溶肥料使用、储存具有指导意义。同时为快速获得聚磷酸铵其他体系相图，引入电解质溶液局部组成模型回归了实验体系的溶剂(盐)盐相互作用能量参数，通过回归参数计算所得的相图与实验相图吻合较好，可用于其他聚磷酸铵体系相图的预测。

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.