Abstract: Ammonium polyphosphate (APP) was prepared with ammonium dihydrogen phosphate and urea as raw materials.Effect of impurities,such as iron,aluminum,and magnesium in raw materials on polymerization degree of APP was studied.Results showed that polymerization degree of APP was only 18,when the raw materials were pure;iron and magne-sium can play positive roles in polymerization of APP when their mass fractions were in the range of （1~1.5）×10<SUP>-4</SUP>.Especially,the polymerization degrees of APP were 57 and 32.6 when the mass fractions of iron and magnesium were 1.5×10<SUP>-4</SUP>and 1.5×10<SUP>-4</SUP>,respectively.Aluminum had a large effect on polymerization degree of APP but always played a negative role.When mass fraction of aluminum was 4×10<SUP>-4</SUP>,polymerization degree of APP was just 1.4. 