Abstract: The effects of technological conditions of aerating ammonia in the preparation course of crystal form Ⅱ ammonium polyphosphate (APP Ⅱ) on the properties of the products were studied.APP Ⅱ was prepared with phosphorus pentoxide，diammonium hydrogen phosphate and urea under different technological conditions of aerating ammonia.Crystal forms and impurities were characterized by FT-IR and XRD.Water-solubility and pH of APP Ⅱ were tested as well.The results showed that the technological conditions of aerating ammonia have impacts on purity，water-solubility and pH of APP Ⅱ.The optimal conditions of aerating ammonia are： initial temperature to aerate ammonia at about 250 ℃；maintain ammonia pressure at 0.1 MPa；final crystal form transformation and amination at 300 ℃ for 90~150 min.The water-solubility of APP Ⅱ is not only relative with crystal forms，but also impacted by the state of ammonia in APP.