Abstract: Ammonium polyphosphate（APP） was synthesized with phosphoric acid and urea as raw materials.Effects of mix ratio of raw materials，polymerization temperature，solidification temperature，and solidification time on performance of product were discussed.Optimum technological conditions were determined as follows：the amount-of-substance ratio of urea to phosphoric acid was 1.7∶1，polymerization temperature was 160 ℃，solidification temperature was 240 ℃，and solidification time was 160 min.Under those conditions，mass fraction of phosphorus pentoxide was 68.55%，mass fraction of nitrogen was 13.0%，average degree of polymerization was 35，pH was 5.45，and flame retardant rating was 65.10%.Product quality accorded with the Industrial APP Standard  (HG/T 2770—2008).

Key words: ammonium polyphosphate, fire retardant, polymerization degree