Abstract: Ammonium polyphosphate with high polymerization degree was synthesized under programming controlled temperature and atmospheric pressure.The structure and performance of products were characterized.The optimized formula was as follows：n（phosphorous pentoxide）∶n(diammonium phosphate）∶n（urea）∶n（melamine）=1∶1∶0.3∶（0.02~0.14），reactive time was 300 min，and reactive temperature was 290 ℃.The results of XRD and IR showed that the product was type-Ⅱ ammonium polyphosphate；the average polymerization degree measured by viscosity method approached to that of Exolit 422 APP of Germany Clariant；the mass fraction of P2O5 was 74.88%；the solubility in water was only 0.455 g（per 100 g H2O)；and the thermogravimetic analysis showed the initial decomposition temperature of this product also approached to that of Exolit 422 APP.

Key words: ammonium polyphosphate, phosphorous pentoxide, melamine, high degree of polymerization