Ammonium polyphosphate (APP) with crystalline form V (APP-V) was prepared through the reaction of ammonium dihydrogen phosphate and urea at high temperature and under dry ammonia atmosphere. Effects such as raw material ratios, ammoniation, condensation temperature, and condensation time were examined. Its water-solubility was tested and compared with those of industrial APP-I and APP-II. FTIR, XRD, and TGA techniques were used to characterize this product. The thermal degradation of APP-V was examined and compared with that of APP-I and APP-II; the molecular structure of this APP-V was speculated and discussed.