Ammonium polyphosphate’s (APP) molecular formula is(NH4PO3)n, n>1000. As inorganic chain polymer, there’s P-N collaboration effect because of high phosphorus and nitrogen content, so it has excellent fire retardant effect. It has good heat stability (decompounding temperature more than 275℃), low water-solubility (no more than 0.2g/100ml water), and it approaches neuter polarity. It becomes the most important and widely used acid and gas stuff in chemistry expansive system. It can be compounded with other fire retardants, and it is of slight toxicity, (LD50>10g/kg). It can be applied in PE, PP, engineering plastic, rubber, fibre after surface modification. So it is named environmental fire retardant.