P4O10 is highly deliquescent and reactive; techniques using non-P4O10 process are not only required for environmental protection, but also for consideration of safety and cost in industrial-scale production. A non-P4O10 process to prepare ammonium polyphosphate with crystalline form II (APP-II) was proposed by heating the mixture of diammonium hydrogen phosphate (DAP) and melamine under wet ammonia. Its water solubility, FT-IR, XRD, and P-31 NMR spectra were compared with those of commercial APP-II and melamine polyphosphate (MPoly), and interesting results were obtained. Thermogravometric analysis (TGA) showed the synthesized APP-II had better thermal stability than commercial APP-II. The reaction mechanism and molecular structure for the subject compound were investigated and discussed in detail.