The invention relates to a method for synthesizing a monomolecular 
phosphorus-
nitrogen expansion 
flame retardant, which is characterized by: reacting 
pentaerythritol and 
phosphorus oxychloride at 80-120°C for 2-12 hours, adding the first 
solvent during the reaction, and the reaction ends  After cooling, filtering, washing, and 
vacuum drying for 12-24 hours to obtain a white 
solid, the white 
solid is 1-oxo-4-
hydroxymethyl-1-phospho-hetero-2,6,7-trioxabicyclo  [2, 2, 2] 
octane (PEPA); then add the white 
solid (PEPA) into the second 
solvent, stir rapidly to dissolve it, then add 
isocyanate substances dropwise, and add the catalyst dropwise at the same time, stirring continuously, the reaction  The time is 1 to 12 hours, the 
reaction temperature is 20 to 100°C, the solution changes from clear to turbid, the product is poured into 
ice water, an appropriate amount of inorganic acid is added dropwise, the pH value is adjusted to be acidic, a white solid is precipitated, and then washed with a washing 
solvent for 3  The filtered product was vacuum-dried to obtain a white or light yellow solid with a yield of 90-96%.  It has the advantages of no 
halogen, non-
toxicity, 
environmental protection, good application prospects and the like.