Abstract: Preparation methods of ammonium polyphosphate (APP)   were introduced.Preparation methods of APP (Ⅰ) and APP (Ⅱ),such as polyphosphoric acid method,phosphoric acid method,ammonium phosphate thermal polycondensation method,and ammonium phosphate and phosphorus pentoxide synthesis method,were emphatically expounded.The degree of product′s polymerization was low by the methods of polyphosphoric acid,phosphoric acid,and ammonium phosphate thermal polycondensation and their main products were APP (Ⅰ).The degree of product′s polymerization was higher by ammonium phosphate and phosphorus pentoxide synthesis method and its product was mainly APP (Ⅱ).Finally,application and developing trend of APP in fertilizer and flame retardant fields were summarized.