The invention relates to a preparation method of a flame retardant nylon material. The preparation method is characterized by comprising the following specific steps: uniformly mixing raw materials including 58.79-72.56 parts by weight of nylon chip, 12.61-18.92 parts by weight of melamine, 11.53-17.29 parts by weight of phosphoric acid and 3-5 parts by weight of charring agent; then putting the mixture into a double-screw extruder; and finally extruding and pelleting so as to obtain the flame retardant nylon material. A flame retardant melamine polyphosphate (MPP) is formed in situ in a melt blending process of the two flame retardant components and nylon. The MPP generated by the method can be uniformly dispersed in a nylon matrix phase at the nanometer size, so that the flame retardant efficiency is improved very well. The prepared flame retardant nylon material has excellent flame retardant property, and the mechanical property of the flame retardant nylon material is hardly reduced.