The invention relates to a
phosphorus-
nitrogen intumescent flame retardant and a preparation method thereof. The preparation method comprises the steps: mixing
neopentyl glycol and a
solvent, adding a catalyst, dissolving the catalyst, then, adding
phosphorus oxychloride into a reaction solution under the condition of stirring at the temperature of 0-10 DEG C, reacting for 4-8 hours at the temperature of 20-50 DEG C after
phosphorus oxychloride is added completely, removing the
solvent through distilling after reacting ends, and washing and
drying residues, so as to obtain cyclic
phosphoryl chloride; adding the obtained cyclic
phosphoryl chloride and an acid binding agent into a
solvent, stirring to dissolve, adding
diamine into a reaction solution under the condition of stirring at the temperature of 15-30 DEG C, then, reacting for 8-12 hours at the temperature of 50-110 DEG C, removing the solvent through distilling after reacting ends, and washing and
drying residues, thereby obtaining the phosphorus-
nitrogen intumescent flame retardant. The phosphorus-
nitrogen intumescent flame retardant and the preparation method thereof have the advantages that the synthesis process is simple, the reaction time is short, the product has a
melting point, the transparency is good when the flame retardant is added into materials, the flame-retardant effect is good, the influence on mechanical properties is low, and the like; and meanwhile, the
disadvantage that intumescent flame retardants cannot be applied to the flame retardance of
engineering plastics due to poor
thermal stability is overcome.