The invention belongs to the field of flame retardants, and especially relates to a reaction type nylon flame retardant and a preparation method. The technical scheme is characterized in that the halonge-free phosphorus-containing nylon flame retardant is synthesized by a two-step process, a first step is preparation of a flame-retardation esterification liquid, the formula comprises 5-52 parts by mass of a halogen-free phosphorus-containing copolymerization type flame retardant and 20-65 parts of a dihydric alcohol, and the flame-retardation esterification liquid is obtained by reacting the above two compositions under a certain condition; and a second step is preparation of the halogen-free phosphorus-containing nylon flame retardant, based on the mass quantity of the flame-retardation esterification liquid, the formula comprises 18-60 parts of the flame-retardation esterification liquid and 5-48 parts of a dibasic acid, and the halogen-free phosphorus-containing nylon flame retardant is obtained by reacting the above two compositions under a certain condition. The obtained flame retardant is resistant to a high temperature of 250 DEG C, satisfies the polymerization temperature of nylon, is polymerized onto a nylon molecular chain in a monomer form in a nylon polymerization process, and has persistent flame retardation effect. The prepared nylon flame retardant has the limit oxygen index of 30% or more, has excellent flame retardation performance, is applicable to produce flame-retardant nylon with the flame retardation performance of UL94V-0 grade or more, and is an efficient environment-friendly nylon flame retardant.