The present invention relates to a method for preparing amino phosphate flame retardants. Melamine and phosphoric acid substances are used as a starting material, AND then the starting material reacts in the absence of solvents at a temperature ranging from 40 to 200° C. to obtain powdery amino phosphate compounds used as flame retardants. Since the reaction free of solvents is carried out under a condition of lower temperature, the powdery amino phosphate compounds can be obtained without drying steps. Therefore, not only the contaminations caused by solvent volatilization can be avoided, but also the process of preparing the product is simplified and the cost of the product is reduced.