The invention relates to a nitrogen-phosphorus flame retardant additive hexa-(DOPO-hydroxymethyl phenoxy)-cyclotriphosphazene (PPC)-based halogen-free flame-retardant epoxy resin and a preparation method thereof, and belongs to the technical field of adding a specific flame-retardant component into the conventional epoxy resin to obtain a flame-retardant composite material. The method comprises the following steps of: performing melt blending on polyphosphazene-DOPO flame retardant PPC and glycidyl ether type epoxy resin at a certain temperature; adding an organic solvent to prepare a flame retardant epoxy resin; and adding a curing agent into the epoxy resin to volatilize the organic solvent at a certain temperature and cure the epoxy resin so as to obtain a flame-retardant epoxy resin cured product, wherein when the content of phosphor reaches 1.2 percent, the flame-retardant level of the product reaches UL94V-0 level.