The invention relates to a trine expansion type flame retardant and a synthetic method thereof. The synthetic method is characterized by comprising the following steps: adding a solvent into a container, adding cyanuric chloride and 1-oxyphospha-4-hydroxymethyl-2,6,7-trioxabicyclo[2.2.2] octane into the container, cooling to 0-45 DEG C in water bath or oil bath and stirring and dispersing, dropwise adding an acid-binding agent to react step by step, thereby obtaining unary substitute; adding matters containing aliphatic diamine or aliphatic dihydric alcohol and a mixture of the solvent and the acid-binding agent into another container, cooling to -10 to 10 DEG C in ice bath or a freezing reactor, slowly dropwise adding a phosphorus-containing compound to react, thereby obtaining phosphorus-containing intermediate; adding the obtained unary substitute (CNC-PEPA) and the obtained phosphorus-containing intermediate into a reaction container with the solvent, dropwise adding the acid-binding agent, heating to 80-130 DEG C to react, cooling, washing and drying to obtain powder solid, namely the trine expansion type flame retardant. The trine expansion type flame retardant has the advantages of being great in molecular weight, relatively moderate in proportion of an acid source, a carbon source and a gas source in a molecular structure, relatively high in char forming content, relatively good in thermal stability and the like.