Wet process purification of phosphoric acid and industrial grade urea are used as raw materials. Through orthogonal experiments, the effects of urea to phosphoric acid molar ratio, wet process purification of phosphoric acid impurity coefficient, pre-polymerization temperature, prepolymerization time, curing temperature, curing time and other factors on the average polymerization degree, P₂O₅ and N content of ammonium polyphosphate are studied. The results show that the optimal process conditions are when the molar ratio of urea to phosphoric acid of 1.8∶1, wet purification phosphoric acid impurity coefficient of 0.003 84, pre-polymerization temperature of 145 ℃, prepolymerization time of 17 min, curing temperature of 235 ℃, and curing time of 2 h. Under optimal process conditions, the obtained ammonium polyphosphate product is a white solid, meeting the requirements of Class Ⅱ first-class products in the industry standard " Ammonium Polyphosphate for Industrial Use" (HG/T 2770—2020).