Abstract: The solubility of (NH₄)₃PO₄in NH₃·H₂O solution is measured in the range from 10 ℃ to 70 ℃ by the method of isothermal solution saturation. Solubility data of the system is determined experimentally, and corresponding phase diagrams are constructed on the basis of solubility data. The results show that the solubility of (NH₄)₃PO₄ declined with increasing of ammonia concentration and decreasing of temperature. For the same ammonia concentration, the solubility of (NH₄)₃PO₄declines with the decrease of the temperature. Removing potassium and sodium from industrial grade ammonium phosphate can be achieved by increasing ammonia concentration and decreasing temperature in the solution. By this way, ammonium phosphate crystal separated from the solution, while potassium and sodium ion remain in the solution, so that ammonium phosphate can be purified.