The invention relates to a phosphorus-nitrogen intumescent flame retardant and a synthesis method thereof. The synthesis 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 synthesis 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.