An efficient synthesis of the reactive phosphorus flame retardant had been developed based on triazine. The stepwise synthesis of the functionalized compound 7 was presented. The chemical structure of the product was investigated with 1H NMR and elemental analysis while the thermal stability was studied by TG analysis. The reaction mechanism of 7, which was synthesized from the compound 4 and 6, was investigated using the density functional theory (DFT) of quantum chemistry. At the GGA-PW91/DND level, the geometry optimizations of the reactant and the product were performed, which showed that the calculated results was agreed with that of the experiment.