The invention provides a preparation method of MPOP. The method comprises steps as follows: (1) preparation of an intermediate: urea phosphate is dissolved in water, the concentration of urea phosphate is controlled to range from 10% to 50%, a certain amount of melamine is added to react with urea phosphate, the molar ratio of urea phosphate to melamine is 1:(0.8-1.25), the reaction temperature is controlled within 25-100 DEG C, the reaction time is 0.5-2.5 h, filtration, washing and vacuum drying are performed, and the intermediate is obtained; (2) preparation of MPOP: the intermediate prepared in Step A is calcined at the temperature of 200-400 DEG C for 1-5 h, cooling, grinding and sieving are performed, and a flame retardant, namely, MPOP is obtained. The technology is simple, industrialization is easy to realize, the operation condition is mild, water is taken as a reaction medium, and the method is clean and pollution-free.