The invention provides a synthesis method of a novel reactive bicyclophosphate flame retardant. The optimal technological conditions for synthesizing the intermediate pentaerythritol bisphosphorate diphosphoryl chloride (PDD) from phosphorus oxychloride and pentaerythritol are as follows: a certain amount of dichloromethane is used as a solvent, a certain amount of triethylamine is used as an acid-binding agent, the mole ratio of the reactants pentaerythritol and phosphorus oxychloride is 1:2.1, the reaction temperature is 50 DEG C, and the reaction time is 6 hours. The optimal technological conditions for synthesizing the target product pentaerythritol bisphosphorate diphosphoryl chloride propenol salt (PDA) are as follows: the mole ratio of the reactants PDD and propenol is 1:2.2, and the reaction is carried out for 8 hours under ice bath conditions. The method has the advantage of simple technique, and is green and environment-friendly. The novel flame retardant synthesized by the method has the advantages of no halogen, no toxicity, high heat stability and the like, can well overcome the defects of low flame-retardant efficiency, high migration tendency, poor compatibility with high-molecular materials and the like in the additive-type phosphorus flame retardant, conforms to the environmental requirements which are advocated at present, and has wide development prospects.