A spiro and caged bicyclic phosphate flame retardant named as PDD-PEPA was synthesized with pentaerythritol diphosphate dichloride (PDD) and caged bicyclic phosphate 1-oxo-1-phospha-2, 6, 7–trioxabicyclo [2.2.2]-4-hydroxymethyl octane (PEPA) as reactants, and the synthesis conditions of the flame retardant, namely the dosage of triethylamine as a catalyst and the reaction time, wereare optimized. Then, the structure of PDD-PEPA was characterized by means of MS, IR and NMR, and the thermal stability and charring capacity of PDD-PEPA were investigated in N2 atmosphere through thermalgravimetric analysis. The results show that (1) when PDD- and PEPA reacts with a mole ratio of triethylamine to theoretical hydrogen chloride of 1:2 in the acetonitrile solvent at a reflux temperature for 10 h, the yield of PDD-PEPA is higher, and (2) under such conditions, the theoretical phosphorus content of the flame retardant is 21.2%, the initial decomposition temperature is 351℃, and the carbon yield is 63.6% at 650℃.