Monoammonium phosphate（MAP），urea and magnesium sulfate were fully mixed and ground to feed into reactor for making ammonium polyphosphate（APP） which contained chelated magnesium by one step feeding method.The effect of magne sium content，urea/MAP amount of substance ratio，reaction temperature and stirring speed of reactor on the polymerization degree and distribution of polymerization degree of the products were investigated.Results showed that the polycondensation between MAP and urea was promoted when the magnesium content was less than 5%.The polymerization degree of APP was significantly increa sed by augmenting the urea proportion and increasing reaction temperature.With the increase of reactor stirring speed，the po lymerization degree of APP containing magnesium increased，while that of pure APP decreased.The order of various exper imen tal factors effecting on APP polymerization degree were the amount of substance ratio of urea/MAP>reaction temperature>stirring speed>magnesium content.